How a Commercial Airliner Starts (Boeing 737-800)

This is the first in a series of articles that explore the inner workings of modern commercial aircraft. In this article, we will walk through the process of getting a Boeing 737-800 aeroplane ready to start moving from a cold and dark state (where everything is powered off). Along the way, we'll discuss various systems on board modern aeroplanes, what they are used for, how they work, and how everything integrates together to allow a giant hunk of metal to fly.

Since aeroplanes are expensive^{[citation needed]}, I'll be demonstrating everything using the Zibo 737-800 mod for X-Plane 11. The article has lots of images, most of which are hidden inside annotations. Click the (1) buttons to view the image, then click the image to enlarge the view. Most sections are followed by a short video clip which demonstrates the steps discussed in that section. Turn on closed captions for the videos to see the steps being carried out in the subtitles.

This is an annotation.

You can contact me regarding any suggestions, criticism, queries or general feedback by writing an email to [email protected].

Important Notice

This is absolutely NOT a guide for operating real-world aircraft or flight simulators. Several key sections which don't fit the theme (many of the safety tests & FMC setup) have been omitted from the article.

1. Aircraft Overview

Before we jump into the pilot seat, let's familiarise ourselves with the aircraft, from inside and from outside. The Boeing 737-800 is part of the 737 NG (Next Generation) series, launched in 1994. It's a narrow-body jet with seating space for around 180 passengers (varies across airlines and configurations). First, let's examine some important components of the aircraft from the outside.

Cockpit - Pilots sit here and operate the aircraft. The 737-800 has seating room for two people in the cockpit, but can include up to two additional jump seats for additional crew or authorized observers.(1)
Landing gear - The nose gear is at the front of the plane, and is used for steering the aircraft on the ground. The main gear is further back, and contains two sets of wheels which bear the impact of the plane after landing.(1)
Cabin - This is where passengers sit, or cargo is stored in a freighter variant like the 737-800BCF.(1)
Engines - The 737-800 has two CFM 56-7B turbofan engines. They use fuel to spin large fans which push air back to generate thrust.(1)
Wings - Wings are designed to generate lift to enable the plane to fly. They also contain fuel tanks.
Tail - The tail section of the plane contains the horizontal and vertical stabilizer, which are discussed in detail in the next section.

All aircraft have various moving surfaces, which allow pilots or the flight computers to precisely control the speed, pitch, heading (direction) and banking of the aeroplane. These are commonly called 'control surfaces'. Here's a brief overview of the control surfaces on a 737-800.

Ailerons - These are located on both wings, and allow the plane to bank (roll left or right). While banking, one of the ailerons flips up, impeding airflow. The other aileron flips down, increasing lift slightly. The plane thus rolls with the wing containing the down-flipping aileron rising higher.
Elevators - Elevators control the pitch of the plane. When they flip down, the horizontal stabilizer generates more lift, pushing the tail up and the nose down. When they flip up, the tail generates less lift, which pitches the plane up.
Rudder - The rudder is used to control the plane's yaw (heading / direction). It flips left or right, and operates on the same principle as the ailerons and elevators. It's usually not used during flight (except by a system called the yaw damper which is discussed later), and is mostly used against crosswinds during takeoff or landing.
Horizontal stabilizers - The entire horizontal stabilizer assembly can rotate slightly upwards or downwards, in a process called 'trimming'. This allows pilots to set a desired pitch using the elevators, and trim the stabilizers so the plane maintains its pitch even when the pilots release the control column to prevent fatigue.
Trailing edge flaps - These extend out from the back of the wing, increasing the wing's surface area. This increases lift, allowing the plane to take off at a slower speed, and also increases drag, helping the plane to slow down before landing.
Flight spoilers - These are panels (4 on each wing) that flip up to disrupt airflow. During banking, flight spoilers on one wing get raised slightly with the corresponding aileron. They can also act as speed brakes, to help the aircraft descend without gaining airspeed.
Ground spoilers - These are panels similar to flight spoilers (2 on each wing), but are located closer to the fuselage. These are strictly deployed (along with flight spoilers) only when the plane lands to create drag and slow it down.
Kreueger flaps - These are smaller flaps on the leading edges of both wings, and perform the same function as the trailing edge flaps.
Leading edge slats - These are protrusions on the leading edge which slide forward and downwards to create a gap, allowing high pressure air from under the wing to flow through the gap and over the top. This reduces the risk of a stall at high angles of attack (pitch).

As to how an aeroplane actually flies, that's a topic reserved for the next article in this series. For now, let's head inside the cockpit and understand the layout of the various instruments and controls.

Left forward panel - Contains the Captain's Primary Flight Display and Navigation Display.(1)
Glareshield panel - Contains switches for configuring automatic flight, and the master caution & fire warning lights.(1)
Control column (First Officer) - The column can be pulled or pushed to change the plane's pitch. The control wheel can be rotated to bank the aeroplane.(1)
Center instrument panel - Contains the engine / fuel display, controls for the landing gear, and a standby attitude indicator and compass.(1)
Control stand - Contains throttles for adjusting engine power, and levers to adjust control surfaces like flaps and spoilers.(1)
Aft electronic panel - Mainly contains panels for communication with air traffic controllers, as well as controls for fire extinguishers.(1)
Forward overhead panel - Contains switches for most electronic, hydraulic, pneumatic and fuel systems. The frontmost section is dedicated to the various lights installed around the plane.(1)
Aft overhead panel - Contains instruments which don't require attention for most of the flight, like the navigation system, cockpit voice recorder and warning tests.(1)
Forward electronic panel - Contains the control display unit for the flight management computer (FMC).(1)

2. Powering the Aircraft

When we enter the aircraft, it is essentially a 70-ton paperweight. Just like any parked vehicle, a parked aircraft has all systems powered off to save fuel and prevent battery drain. This is commonly called the 'cold and dark' state. We first need to establish a stable electrical source to power the aircraft and its instruments. Similar to consumer vehicles, aeroplanes have a battery to power their electrical systems before the engine starts. The B737-800 has a 24 volt nickel-cadmium battery in the electronics compartment(1) (optionally an auxiliary battery as well). A single battery can provide standby power for at least 30 minutes. So we first flip the battery switch.(2)

PaulGreasley, CC BY-SA 4.0, via Wikimedia Commons

The battery provides power to many essential systems on the aircraft, like fire detection and suppression systems for the engines, fuel shutoff and cross-feed valves, and (in case all other systems are offline) the flight management computer(1). However, the battery alone cannot power all electrical systems on the aircraft, like most cockpit displays and flight computers, cabin lighting or electric pumps to pressurise the various hydraulic lines, and more importantly, it doesn't last very long. We require a higher capacity power source.

The batteries cannot power the FMC directly, since it requires alternating current. The FMC instead draws power from the standby bus, which is connected to the batteries via the static inverter that can convert DC into AC.

Ground Power

The GPU (Ground Power Unit) is a generator which sits on the ground and provides electrical power to the aircraft via a cable(1). After we switch on the battery, if a GPU is connected, the GND POWER AVAILABLE light (on the Ground Power panel) will be illuminated. We can then push the spring-loaded toggle switch down which connects the GPU to the AC transfer buses.(2)

We also flip the POSITION light switch to STEADY(1). This turns on the navigation lights on the wingtips(2) (red + white on the left wingtip, green + white on the right), which indicate to the ground crew that the plane has electrical power and is occupied. The coloured lights on each wingtip face forward (towards the nose), while the white lights face backward. This helps other planes in the air know whether the aircraft is moving towards them or away from them based on what lights they see.

In some cases, ground power may not be available, in which case we use the APU.

Auxiliary Power Unit

The APU (Auxiliary Power Unit) is a small engine inside the tail end of the plane(1). It acts as a generator, consuming the aircraft's fuel and producing electricity to power the various flight systems. If we are connected to ground power, we can start the APU much later, before the aeroplane is pushed back from the gate.

Before we can start the APU, we must ensure that the aircraft can warn us if something goes wrong (the APU overheats or ignites). The 737 has a fire detection loop for the APU with a detector that can sense a rise in temperature. We can test the continuity of this loop and two other loops for the engines by using the TEST switch on the Overheat/Fire Protection panel.

We first move the switch to the left, i.e. FAULT/INOP, which tests the fault detection circuits for the APU and engines(2). The MASTER CAUTION, OVHT/DET, FAULT, and APU DET INOP (APU Detector Inoperative) lights should be illuminated.(1)

Then we move the switch to the right (to OVHT/FIRE) to test the overheat and fire detection loops on the APU, engines, and the wheel well (space in the underbelly where the retracted wheels are stowed)(2). This time, the fire warning bell should ring, with the FIRE WARN, MASTER CAUTION and the OVHT/DET lights illuminated, along with the three fire warning switch lights.(1)

If this test is performed with the GPU active, the WHEEL WELL light activates as well.

We're not yet done with the tests. The APU and engines have fire extinguishers which can be triggered by small explosive devices (squibs). We test the continuity of the two extinguisher circuits by toggling the EXT TEST switch to the left and right, which should light up all three lights below it (one for each engine and the APU).(1)

These tests can be performed on battery power in the absence of ground power.

We are now ready to start the APU. On the forward overhead panel, we flip the APU switch to ON, then push it down momentarily to START(1). This initiates the APU start sequence. The air inlet doors near the tail open to allow airflow into the APU. A starter generator near the APU (powered by the GPU or battery) spins the APU up to a target speed, then fuel and ignition are provided. The exhaust gas temperature (EGT) initially ramps up to around 800 °C due to excess of fuel before the airflow is fully established, then settles down to 400 °C as the APU stabilises. When the APU is fully initialised, the APU GEN OFF BUS light illuminates (indicating that the APU generator is running and not powering a bus). Now we push the spring loaded APU GEN switches down(2). This connects both AC transfer buses to the APU generator.

The aircraft is now powering itself.

3. IRS Alignment

Now that we have power for the aircraft, one of the first things we need to do is start the alignment procedure for the aircraft's IRS (Inertial Reference System). The IRS is a device used to detect the plane's heading (direction), acceleration, vertical speed, attitude (pitch), and other crucial information. The IRS is extremely precise (we'll see how later), and the only source of attitude and heading information (apart from a standby attitude indicator & a compass).

The Boeing 737-800 has two IRSs, each containing three sets of laser gyroscopes and accelerometers. In simple terms, the laser gyroscopes allow the IRS to sense the Earth's rotation. It can "observe" the axis of the spin, and thus determine the True North pole. It can also determine the plane's latitude based on the intensity of rotation, as the Earth's surface rotates quicker near the equator than the poles.

How do laser gyroscopes work?

A ring laser gyroscope (RLG) operates on the principle of the Sagnac effect. Consider the setup on the right, where a laser source S emits a beam that hits a half-silvered mirror at point J, causing the beam to split and head towards points K and M. The beams eventually reach the detector D where they interfere and produce a fringe pattern. One of the beams thus follows the red path (clockwise), while the other follows the blue path (counterclockwise). If the loop (JKLM) starts rotating clockwise, the counterclockwise rotating beam will have a shorter distance to travel, thus shifting the fringes on the interference pattern.

An active ring laser gyroscope, which is commonly used in aeroplanes like the 737-800 consists of a chamber made of ceramic glasses like CerVit or Zerodur, which have ultra-low thermal expansion (don't deform when temperature changes). This chamber has a cavity filled with gases (usually Helium and Neon isotopes) called the gain medium.

High voltage is applied across the medium, which produces light within a small range of frequencies. This light travels around the cavity repeatedly in both directions, and interferes with itself. Most of the frequencies, which are not a multiple of the chamber length, interfere destructively and die out. However one of the wavelengths is at the correct frequency to produce resonance within the chamber, and thus produces standing waves that get amplified dramatically and "lase" inside the medium. When the gyroscope is stationary, the light traveling clockwise and counterclockwise has to travel the same distance, and thus has the same frequency. However, when the gyroscope rotates, one of the beams has to travel a shorter distance, and the other one has to travel a longer distance. From the perspective of the beams, the cavity length has changed, and thus the frequencies which were producing resonance earlier won't anymore, instead destructively interfering and dying out. Thus, the light moving in each direction starts resonating at a different frequency which is capable of resonating at the changed cavity length. These two frequences overlap and add up at the interferometer, where they produce a beat frequency proportional to the angular velocity. To the interferometer, this looks like a fringe pattern (dark and light bands of light) which moves across it at a certain speed. It can measure the time between two successive bands of light, and thus determine the beat frequency and the angular velocity of the gyroscope.

As you can imagine, the IRS is extremely sensitive to external vibrations, and thus it is vital that the plane remains stationary during its initial alignment, which can take around 10 minutes. On the IRS mode selector unit (aft overhead panel), we turn the IRS mode selector from OFF to NAV(1). On the way, we pass by ALIGN, which initiates the alignment cycle. The ON DC light turns on briefly as the IRS performs a self-test. Then the white ALIGN light comes on, indicating the alignment is in progress. The IRS display above this panel shows the time required for alignment in minutes, if the knob is set to HDG/STS.

The IRS can also operate on battery power. In such cases, the amber ON DC light will be illuminated.

The IRS can determine the latitude, but it can't determine the longitude since the rotation speed of the Earth is the same at every point on the longitude. To assist the IRS, we enter the coordinates of the current airport into the flight management computer. Most of the FMC programming is beyond the scope of this article, but you can read the collapsible block below if you're interested.

Setting IRS position in the FMC

The Control Display Unit for the FMC is present on the forward electronic panel. We click on the button next to FMC to access the flight management computer(1). This screen shows us the aircraft model and some other information like the navigation database expiry date(2). We click on the button next to POS INIT to go to the position initialization page(3). Here, the line below SET IRS POS contains blank squares. As a convention, such fields are mandatory and must be filled out for setup. Other fields like the REF AIRPORT and GATE which have dashed lines underneath can be filled, but aren't mandatory. Rather than filling out our coordinates manually, we can use the FMC's internal database. We type the 4-letter ICAO code (KSEA in our case for Seattle-Tacoma International Airport) which gets put into the scratchpad (bottom line of the screen). We press the button below REF AIRPORT to transfer the scratchpad text into that field(4). Now, the CDU displays the airport coordinates next to the name. We press the button to the right to copy these to the scratchpad(5). Finally, we press the button next to the empty boxes to paste the airport coordinates into SET IRS POS. If we failed to set the position here, the IRS would never align, and the ALIGN lights on the aft overhead panel would start flashing.

After alignment is complete, the ALIGN lights will extinguish. However we can continue with our setup while the IRS is aligning itself.

4. Preparations for Engine Start

Yaw Damper

Aircraft are designed to be stable during flight, and automatically correct for disturbances (like turbulence) which can temporarily alter the orientation of the aircraft. For instance, a gust of wind can push the plane sideways, causing it to yaw. When this happens, the vertical stabilizer (tail fin) is no longer pointing directly towards the oncoming airstream, and instead creates a sideways lift, pulling the tail and the plane back into the airstream. This is similar to a weather vane, which always orients itself to the direction of the wind. Similarly, the wings on many commercial aeroplanes are not perpendicular to the ground; instead forming a V-shape(1). If the aircraft rolls to one side, one of the wings moves downwards. This wing is now much more in line with the airstream than the other, and thus generates more lift. This causes the wing to be pushed upwards, rolling the plane back straight. All this sounds good in practice, but there's one issue caused if the aircraft has a tapered leading edge (like the swept-wing design found on most planes)(2).

When the aircraft yaws to one side, one of the wings turns into the oncoming air. The leading edge of this wing is more in line with the airflow than the other, which causes it to generate more lift and roll the plane. All this time, the vertical stabilizer is trying to restore the yaw, but it's weaker than the massive wings. By the time it manages to point the aircraft back into the airstream, the plane's momentum causes it to overshoot and yaw the other way. Now the leading edge of the other wing is pointing into the oncoming air and starts producing more lift, causing the aeroplane to roll the other way. This cycle continues, causing the aircraft to oscillate in a wagging and rocking motion called a Dutch roll(1).

Picascho, Public domain, via Wikimedia Commons

To prevent this, most aircraft (especially ones with a swept-wing profile) have a yaw damper which makes small adjustments to the rudder to counteract this oscillation and increase the aircraft's stability in flight. On the 737-800, the yaw dampers (main and standby) are controlled through the Stall Management / Yaw Damper (SMYD) computers. These computers receive input from the ADIRU (Air Data Inertial Reference Unit). While cruising, pilots rarely touch the rudder foot pedals. In fact, the pilot's rudder input is dampened in proportion to the aircraft's speed, so if the pilot presses down on the pedal slightly too hard, the aircraft doesn't spiral out of control, or the vertical stabilizer doesn't snap off due to the wind. We turn on the YAW DAMPER switch on the overhead panel, and the amber YAW DAMPER light extinguishes.(1)

Fuel Pumps

We also turn on the fuel pumps. Most commercial aircraft have three fuel tanks - one in the belly and one in each wing. On the 737, each fuel tank has two fuel pumps. The center tank has left & right pumps, while each main tank (in the wings) has a "Forward" and an "Aft" pump. The aft pumps are near the rear and at a lower height than the forward pumps. These pumps are AC powered and are cooled and lubricated by the fuel passing through them. The center pumps produce a higher pressure than the main pumps, ensuring that the center tank is drained first even when all fuel pumps are switched on.

Depending on how the aircraft has been fuelled for our flight, we switch on the corresponding fuel pumps(1). The amber LOW PRESSURE lights should extinguish as the pumps are turned on.

An interesting design choice to note (you can see this better in the video clip) is that when all the pumps are turned off, the amber low pressure lights for the center fuel pumps are extinguished, while the ones for the wing (main) tanks are lit. The low pressure lights come on under the following conditions.

Main tanks : Fuel pump output pressure is low (also occurs when the pump is switched off)
Center tank : Fuel pump output pressure is low AND the fuel pump is switched on.

Since the center tank is drained first during flight, the pilots can be notified when the tank is empty by the low pressure lights turning on. They can then switch off the fuel pumps, which causes the low pressure lights to extinguish, and the pilots don't have to get distracted by two amber warning lights indicating a condition that they already know about (that the center tank is empty).

If ground power is connected before starting the APU, the left aft fuel pump is usually switched on. If not, the APU draws fuel from the left tank using its own suction pump which runs on the battery and creates a sufficient vacuum to pull fuel from the tank.

Hydraulics

Now we turn on the hydraulic pressure pumps. The 737 has two independent hydraulic systems (A & B), with a third standby system used in the event of a failure in the two main systems. They use a synthetic fire-resistant hydraulic fluid called Skydrol, and power most actuating systems on the aeroplane, including:

Most control surfaces (rudder, ailerons, elevators, flaps, spoilers, etc.)
Landing gear, wheel brakes and nose wheel steering
Autopilots

These pumps operate at an incredible pressure of 3000 PSI (for context, a car tire is about 35 PSI). To maintain this pressure, each hydraulic system has two pumps - an AC motor driven pump (which can operate on the APU generator), and an engine-driven pump (operated by one of the engines). We cannot start the engine pumps yet, however we will turn on the two electric pumps. This will provide pressure for the parking brake and nose wheel steering.

We flip on the ELEC 2 and ELEC 1 switches, and the LOW PRESSURE lights should extinguish.(1)

Air Conditioning

At this point in a real aircraft, we would also turn on the air conditioning to regulate the cabin temperature. Along with electric power, the APU also supplies hot compressed air (called bleed air, since it's "bled" / drawn from the APU's compressor). We'll need this bleed air later when we start the engines, but right now we can instead route it through the Pneumatic Air Conditioning Kits (PACKs) which are used to condition the cabin air. In simple language, the PACKs consist of an air cycle machine (ACM), which first pressurizes the incoming air, heating it up. The pressurized air then passes through heat exchangers which remove most of the heat from the air. Finally it enters a turbine, which expands the air rapidly causing it to cool down significantly, often to temperatures below freezing. This air is then mixed with some hot bleed air to achieve the desired temperature, then pumped into the cabin.

To use the bleed air, we flip the center bleed switch down, which opens the APU bleed air valve. We can set the isolation valve to AUTO. The isolation valve isolates the left and right sides of the bleed air duct. Finally, we flip the L PACK and R PACK switches to AUTO.(1)

We would now program the flight management computer (FMC) to set our route, weight and a bunch of other parameters. But since that's beyond the scope of this article, we instead skip to the point where the aircraft is almost ready to be pushed back from the gate. If we were on ground power, now would be a good time to start the APU and disconnect ground power.

5. Pushback and Engine Start

Before starting pushback, we ensure that the ground power is disconnected, all doors are closed and locked, and the parking brake is set. The ground crew use a tractor / tug to push back the aeroplane from the nose wheel(1). Since pilots can't see behind the aircraft, steering it is carried out by the ground crew. They insert a bypass pin into the nose gear to disconnect it from the hydraulic steering mechanism and allow it to rotate freely(2). Engines are often started during pushback.

Hunini, CC BY-SA 4.0, via Wikimedia Commons

Anti-Collision Lights

We also flip the ANTI COLLISION light switch to ON(1). These are red strobe lights on the top and bottom of the fuselage(2). They indicate that engines are running or about to start, and that ground crew & vehicles should maintain a safe distance from the aircraft.

Engine Overview

The Boeing 737 is powered by two CFM 56-7B turbofan engines. At the front of the engine is a large fan which sucks in air. Most of this air is diverted around the engine's body and straight out the rear, and provides majority of the thrust to propel the plane forward. But some of the air enters a low pressure compressor, which rotates with the fan. The air is compressed and moves further into a separate high pressure compressor which is mechanically independent, and compressed even more. At this point, fuel is introduced and ignited, causing the temperature and pressure to build up tremendously. The heated pressurized air then flows to the back, where it pushes through a high pressure turbine (which is connected to & drives the high pressure compressor), and finally a low pressure turbine (which is connected to & drives the low pressure compressor & intake fan) on its way out of the engine. This is how the engine can power itself during flight.

The engine thus consists of two independent rotors. The N1 rotor - consisting of the intake fan, low pressure compressor & low pressure turbine, and the N2 rotor, consisting of the high pressure compressor & high pressure turbine. The N2 rotor is also connected to an accessories gearbox, which is used to drive the fuel, oil and hydraulic pumps, and the Integrated Drive Generator (IDG) which converts the variable gearbox speed to a constant speed for the electric generators.

Engine Start Procedure

To start the engine, a starter motor is connected to the N2 shaft through the accessories gearbox. Once the N2 compressor reaches a sufficient speed, fuel is introduced, which heats up and expands the compressed air rapidly, causing it to move the two turbines on its way out. This creates a feedback loop of the hot air turning the intake fan, which draws in more air to be compressed and heated. Now that we have some preliminary knowledge, let's start the engines on our aircraft one by one.

The engine's starter motor is driven by bleed air from the APU, and we need as much pressure as we can get to drive the large assembly. So we turn off the L PACK and R PACK conditioning units(1). However the bleed air isn't driving the starter motors yet, due to start valves that are currently closed. The order of engines started is usually alternated between flights to prevent excessive wear on any one engine.

Let's turn on the right engine (Engine #2) first. We move the right ENGINE START switch from OFF (or AUTO on some variants) to GRD (ground)(1). This causes APU bleed air to rush into the Engine #2 starter motor, which rotates the N2 shaft. We can see the N2 RPM rise on the lower display unit, represented as a percentage of the maximum operating limit(2). As the N2 (high pressure) compressor begins sucking in and propelling air, it pushes the N1 (low pressure) turbine on its way out of the engine. This causes the giant fan at the front to gently begin rotating. After N2 reaches 25%, the intake fan is pulling in enough air to begin ignition without overheating the engine. Below the thrust levers are the two engine start levers, which control the ignition systems and fuel cutoff valves.

We move the start lever for Engine #2 from CUTOFF to IDLE(1), which allows fuel to stream into the engine, where the igniters (two for each engine) start its combustion. This causes the EGT dial (exhaust gas temperature) to shoot up, and the N2 and N1 dials also rise rapidly(2), as the airflow spins up the low pressure turbine & the intake fan. When N2 reaches about 56%, the engine start switch returns to AUTO on its own. The engine starters stop firing, and the start valve closes. The engine is now up and running, and we can repeat the same procedure for the other engine.

Now that both engines are turned on and generating power, we can switch to the engine generators and take the load off the APU. To do this, we turn back to the forward overhead panel, and on the BUS TRANSFER module where we switched on the APU generators, we now flip the two switches for GEN 1 and GEN 2(1). This disconnects the APU generator from both AC bus and instead connects the two IDGs.

Now we turn off APU bleed, and then flip the APU switch to OFF since we don't need it anymore. We can also turn on the L PACK and R PACK to restore air conditioning(1). We can now turn on the engine-driven hydraulic pumps. We turn on the ENG 1 and ENG 2 switches on the forward overhead panel if they were not already on, and watch the LOW PRESSURE lights extinguish.

We also turn both ENGINE START switches to CONT(1). This causes the engine start switches to fire continuously. This is a precautionary measure taken during takeoff and landing. If the engine combustion stops due to some reason during these critical sections of the flight, the igniters ensure that the engine starts back up immediately.

6. Before Taxi Preparations

Flight Director

The flight director is a part of the aeroplane's Flight Management System (FMS), which is a system responsible for route calculation, in-flight performance optimization, fuel monitoring, and the various cockpit displays. It manages the plane's lateral flight path (LNAV) using an extensive database containing route information, like SIDs(1), STARs(2), waypoints, and holding patterns for various airports. It also manages vertical navigation (VNAV), i.e. controlling the altitude and vertical speed by accounting for altitude restrictions, optimum speeds and fuel burn data. The flight director does the job of showing the optimized flight path calculated by the FMS to the pilots. It usually shows up as a pink / green crosshair on the primary flight displays(3). If the crosshair is off-center, the pilot flying needs to steer the aircraft until it moves to the center.

A SID (Standard Instrument Departure) is the route followed by aircraft immediately after taking off from a runway. The SID can restrain the maximum speed or altitude of the aircraft, and makes it easier for air traffic controllers to manage the flow of aircraft out of an airport.
A STAR (Standard Terminal Arrival Route) is an approach procedure followed by flight crew to land at a particular runway. Like SIDs, STARs are standardized so that aircraft can line up behind each other and land sequentially.
User:Westnest, Public domain, via Wikimedia Commons

There is a companion system to the flight director called the Autopilot which makes this job much easier. The autopilots (there are 2 in most aircraft) get input from the flight director, and physically move the plane's control surfaces (ailerons and rudder) to follow the flight director's commands. The autopilot works with the flight director to keep the plane on course and at the correct altitude.

On the glareshield below the front window, we flip both F/D switches to ON. This turns on the flight directors for the Captain's and First Officer's primary flight displays. The small indicator above the first toggled switch should illuminate MA (Master)(1).

However, the flight director and the autopilot have no control over the part of the aeroplane that actually keeps it going forward - the engines. The engines are controlled by another independent system, the autothrottle.

Autothrottle

The autothrottle controls the engine speed during the various stages of flight. During takeoff, it operates in 'thrust mode', setting the N1 speed so that the engines provide maximum thrust without going over their limit. This 'thrust mode' is usually maintained during initial stages of the climb, where the autothrottle maintains a constant climb power, and the plane's speed is controlled by its pitch (and by extension, the climb rate). In late stages of the climb (as the plane approaches its cruising altitude) and while cruising, the autothrottle operates in 'speed mode', where it tries to maintain a specific airspeed (set by the pilots manually or through the flight plan) by adjusting the engine power. In the 737, the thrust levers have servo motors which are controlled by the FMS. This means whenever the autothrottle makes changes to the thrust, the thrust levers in the cockpit physically move to reflect the change. This provides the pilots with an immediate visual feedback as to what the autothrottle is doing.

We flip the A/T switch to ARM. The indicator light above the switch turns green to indicate that autothrottle can now be turned on during takeoff(1).

Pitot Tubes & Probe Heat

Pitot tubes are part of a system used to measure a plane's indicated airspeed, which is shown by the speed tape on the primary flight display. It's not the "true" speed of the aircraft, since it's measured by reading the amount of air rushing past the aircraft. As a result, even if the aircraft is flying at a constant speed, the IAS can change if there is a tailwind / headwind. Also, air pressure decreases at higher altitudes, and so does the amount of air flowing past the plane. So an indicated airspeed of 200 knots is very different at 2,000 feet and 30,000 feet. However, IAS is still used where aerodynamics matters, like the takeoff speed and stall / overspeed warnings.

Pitot tubes are small cylindrical devices fixed on the sides of the fuselage pointing forward(1). When the aircraft is flying, air rushes into the tubes and cannot get out since the end is blocked. Pressure builds up in the tube up to a limit, based on the aircraft's speed. This pressure, called the 'stagnation pressure' is measured by the FMS and compared to the atmospheric pressure at that altitude measured using static ports (small holes at the sides of the fuselage)(2). The flow velocity can now be determined using a variation of the Bernoulli equation:

\[ u = \sqrt{\frac{2(p_t - p_s)}{\rho}} \]

\(u\) is the flow velocity
\(p_t\) is the stagnation pressure
\(p_s\) is the static pressure
\(\rho\) is the fluid density

The pitot tubes have a small opening at the front for air to flow in. At high altitudes, where the temperature is well below freezing, ice can build up on the tube, disrupting / blocking airflow. To prevent clogging, each tube has a heater element inside which can melt / prevent ice formation. We turn on the heaters by flipping the PROBE HEAT switches to ON(1). The lights should extinguish as the probe heaters are turned on.

There are eight external probes which are electrically heated:

3 pitot tubes near the nose (for airspeed measurement)
2 pitot tubes on the tail's vertical fin (for the elevator feel computer)(1)
2 alpha vanes near the nose (used to measure the angle of attack)
1 total air temperature probe (for measuring external temperature)

Elevator Feel and Centering Unit

The Elevator Feel and Centering Unit uses hydraulic systems to apply artificial pressure on the pilots' control columns. Since the elevators are controlled by hydraulic systems and not mechanically linked to the control columns, the pilots have no feedback of the aerodynamic loads acting on the elevators. The EFCU mimics this force using airspeed from the elevator pitot system and the horizontal stabilizer position. This allows the pilots to have an intuitive sense of the aircraft's response, and helps them make precise inputs while maintaining control of the plane. The EFCU also controls elevator centering. When the pilots let go of the control column, the EFCU ensures that the elevators return to a 'neutral' position.

During a stall, when the aircraft has a high angle of attack and is flying too slow to generate lift, the Speed Trim System (STS) adjusts the horizontal stabilizers to point the nose down to recover from the stall. Another module called the Elevator Feel Shift (EFS) increases the control column force to twice the normal (or up to four times according to some sources). This gives pilots a clear indication that they should not be pulling the column back, and prevents them from overpowering the STS trimming the elevator and trying to gain airspeed.

Emergency Exit Lights

All aircraft have emergency exit lights located throughout the cabin to guide the passengers safely out of the aeroplane in case of an emergency. We set the EMER EXIT LIGHT switch to ARMED on the overhead panel(1). The lights will remain extinguished under normal operating conditions, however if AC power has been turned off or power to the DC bus 1 fails, the lights turn on automatically(2).

Cabin Pressurisation

The air pressure at an aeroplane's cruising altitude (~30,000 ft) is much lower than at the ground. We (humans) require oxygen to breathe which we inhale through the air. At the cruising altitude, even though the concentration of oxygen in the air is same as the ground, since the pressure is much lesser, our blood cannot absorb enough oxygen from the lungs which can lead to hypoxia or unconciousness. To prevent this, and for the general comfort of passengers, the cabin pressure control system regulates the pressure inside the cabin using the PACKs and outflow valves. Essentially, the PACKs provide a constant supply of conditioned air into the cabin (drawn from the engines). The outflow valve is an opening near the tail for air to escape(1). By modulating the size of the opening, the controller can regulate the pressure inside the cabin. During flight, the cabin pressure is purposefully kept lower than sea level, since the pressure difference would be too high for the fuselage to safely handle. This pressure is displayed in the cockpit as the 'cabin altitude'. If the cabin altitude is 8,000 feet, the cabin pressure is equivalent to the air pressure at 8,000 feet above sea level.

The outflow valve is controlled using a DC motor connected to two independent systems (AUTO and ALTN). In the event the valve or the controller fails and air pressure starts building up in the cabin, two pressure relief valves are provided on the 737-800 which limit the maximum pressure difference to 9.1 PSI(1). A negative pressure relief valve prevents external atmospheric pressure from exceeding the cabin pressure.

wsombeck, Public domain, via Wikimedia Commons

On the cabin pressurisation panel, we set the FLT ALT (flight altitude) and LAND ALT (altitude of the landing strip)(1). The flight altitude is used to control the rate of change of the cabin altitude. The controller gradually increases the cabin altitude (i.e. gradually decreases cabin pressure) such that it reaches its target at the same time as the plane reaches its cruising altitude. The landing altitude is used by the controller to calculate a comfortable descent rate for the cabin altitude. Before touchdown, the controller keeps the cabin at a slightly lower pressure than the outside air, to prevent passengers feeling pain in their ears from the sudden pressure change as the outflow valve opens to equalise the pressure.

Autobrake

During takeoff if certain critical aircraft systems fail or malfunction, the pilots may choose to abort takeoff while the plane has already gained considerable speed. In such cases, even with assistance from systems like spoilers and thrust reversers, the plane has to apply a tremendous amount of braking force to stop safely before it overshoots the runway. There are two important reasons for this. First, the plane is loaded with fuel, so it has a much higher weight and inertia than when it lands. Second, depending on when the pilots decided to abort, they may have consumed a significant amount of runway during the takeoff roll. The autobrake system applies pressure to the wheel brakes (through hydraulic system B) to slow the aircraft down during a rejected takeoff, or during landing.

We arm the autobrake system by setting the switch on the center forward panel to RTO (rejected takeoff)(1). The system performs a power on self-test, during which the AUTO BRAKE DISARM light illuminates momentarily and extinguishes. During takeoff, if the ground speed is over 90 knots, and the pilot retards the thrust levers to IDLE, the autobrake system applies maximum braking power (within limits set by the anti-skid system) to the wheels through hydraulic system B.

The last thing we do is press the annunciator button nex to the MASTER CAUTION light. This recalls any warning suppressed using the MASTER CAUTION button. We make sure that there are no warnings on the annunciator before beginning with the taxi.

Conclusion

Congratulations! The aeroplane is now configured for taxi. At this point, Air Traffic Control would give us the route towards the active runway and we could finally start moving. In the next article, we will discuss the systems which come into play while taxi, during takeoff and the initial climb. Thanks for reading! You can contact me regarding any suggestions, criticism, queries or general feedback by writing an email to [email protected].