B737 Normal Procedures: Engine Start, Taxi and Takeoff

The Boeing 737 engine start, taxi and takeoff procedures take the airplane from the gate to the After Takeoff checklist. Each is a flow done from memory and confirmed by a checklist, with most steps tied to a light, an indication or a callout.

This article follows the 737 NG flight crew operations manual (FCOM), chapter NP.21, and says where the 737 MAX differs; an operator's own procedures, built on Boeing's, govern its crews. The pilot flying (PF) taxis and controls the flight path, airspeed and configuration; the pilot monitoring (PM) reads the checklists, communicates and monitors (see pilot flying and pilot monitoring). The procedures before engine start are in B737 preflight and before start procedures, the climb onward in B737 descent, approach, landing and shutdown.

Pushback or towing

Boeing's procedures assume coordination with the ground crew before hydraulic pressurization, flight control movement or airplane movement. Normal nose wheel steering is powered by hydraulic system A. A nose gear steering lockout pin in the towing lever depressurizes the steering, so that the airplane can be pushed or towed without depressurizing the hydraulic systems.

The before start procedure therefore checks the pin when a pushback is needed. If it is not used, system A must not be pressurized, because unwanted tow bar movement can occur: its pumps stay OFF, the system B electric pump goes ON, and the brake and system B pressures must reach 2,800 psi. Otherwise both electric pumps go ON (see B737 hydraulic system).

During the pushback procedure two cautions apply. The crew must not hold or turn the nose wheel steering wheel, and must not use the airplane's brakes to stop it; either can damage the nose gear or the tow bar. When the pushback or tow is complete, the crew verify that the tow bar is disconnected and the lockout pin removed, switch the system A pumps ON, check that their LOW PRESSURE lights are out and that system A shows at least 2,800 psi.

Engine start procedure

The 737's air turbine starter needs pressurized air, from the APU, a ground cart or the other engine, and electrical power. The engine start switch at GRD uses battery power to close the engine bleed valve and open the start valve. The packs are switched OFF for the start, leaving the starter the full duct pressure. The ignition select switch, IGN L or IGN R, is alternated on successive starts; on most variants the left igniter is powered from its AC transfer bus and the right one from the AC standby bus. For the first flight of the day at airports at or above 2,000 ft with a temperature below 5 °C, the crew may consider BOTH to improve the chance of a first-time start.

The engine start procedure on the NG runs as follows:

1. The captain calls "START ENGINE [number]" and sets the ENGINE START switch to GRD. N2 must increase, and the amber START VALVE OPEN alert shows the start valve open.
2. When N1 rotation is seen and N2 reaches 25 %, the engine start lever goes to IDLE. If 25 % cannot be reached, it goes to IDLE at maximum motoring, when N2 gains less than 1 % in about 5 seconds, with at least 20 % N2. The lever energizes the ignition through the EEC and opens the spar and engine fuel shutoff valves.
3. Light-up shows as an EGT rise, which must come within 15 seconds.
4. At 56 % N2 the ENGINE START switch returns to OFF, or is moved there, START VALVE OPEN goes out and the crew call "STARTER CUTOUT".
5. The engine is stable at idle when the EGT start limit redline is no longer shown.

View full schematic

Three warnings frame the start. A start lever moved to IDLE early can cause a hot start. If fuel is cut off by accident with the start lever, the lever is not reopened to try to restart the engine. If the switch fails to stay at GRD until starter cutout, a hot start can follow, and it is not re-engaged until N2 is below 20 %. Each start attempt is limited to 2 minutes of starter use, with at least 10 seconds between attempts; extended motoring is limited to 15 minutes for the first two runs, 2 minutes apart, then 5 minutes, 10 minutes apart.

Aborted start

The ABORTED ENGINE START checklist is done for a hung start, in which N1 or N2 does not increase, or increases very slowly, after the EGT rises; no oil pressure by the time the engine is stable at idle; no EGT rise within 15 seconds of the start lever reaching IDLE; or an EGT quickly nearing or exceeding the start limit. Its only recall item is the start lever to CUTOFF. The EEC also watches ground starts. It flashes the white box around the EGT readout for an impending hot start or a stall, and with EEC software 7.B.Q or later it then cuts the ignition and fuel itself. It also cuts them when EGT exceeds the start limit, which turns the EGT display red, and in a wet start, with no EGT rise, 15 seconds after the lever reaches IDLE. A red warning or amber caution during or after the start calls for the non-normal checklist, and on the ground the dispatch deviation guide.

On the 737 MAX the LEAP-1B engines replace the CFM56-7B, and the FAA FSB grades them as a Level B difference with a procedure change; start figures should come from the MAX FCOM rather than the NG's (see B737 engines and engine indications).

Before taxi procedure

The before taxi procedure starts once the engines are stable at idle. The generators go ON, probe heat ON and wing and engine anti-ice as needed. The packs return to AUTO and the isolation valve to AUTO, the APU bleed and then the APU are switched OFF, and the ENGINE START switches go to CONT, since ignition is required for takeoff and landing, in heavy rain and with anti-ice on. The start levers are checked in the IDLE detent. The flap lever is set to the takeoff flaps, with the green LE FLAPS EXT light on.

The flight controls check uses slow, deliberate inputs, one direction at a time. The control wheel, the column and the rudder pedals go to full travel each way, checking freedom of movement, the return to center and, if the surface positions are displayed on the lower display unit, the correct surface movement. The nose wheel steering wheel is held during the rudder check to keep the nose wheel still. A recall then shows every system annunciator light coming on and going out.

The PF taxis; the nose wheel steering wheel turns the nose wheels up to 78° either way; the rudder pedals give up to 7°, a limited authority deactivated as the nose gear strut extends. CDU entries are made before taxi or when stopped where possible; during taxi the PM makes them and the PF verifies them before execution.

Before takeoff procedure

The engines must be warm: the requirement is an increase in engine oil temperature before takeoff; Boeing recommends running the engines for at least 2 minutes at a thrust normally used for taxi. The pilot who will make the takeoff updates the takeoff briefing for any change, and the weather radar and terrain displays are set as needed. The cabin crew are told to prepare for takeoff, the cabin is confirmed secure, and the Before Takeoff checklist follows.

The last defense is the takeoff configuration warning. On the ground, as either forward thrust lever is advanced for takeoff, it sounds an intermittent horn and lights the red TAKEOFF CONFIG lights if the trailing edge flaps are outside the flaps 1 to 25 range or have a skew, asymmetry or uncommanded motion, if the leading edge devices are not in takeoff position or have moved uncommanded, if the speed brake lever is not DOWN, if the spoiler control valve is open, if the parking brake is set or if the stabilizer trim is outside the takeoff range. The FCOM warns that the cabin altitude warning uses the same intermittent tone. Fuel matters too: the random imbalance between main tanks 1 and 2 may not exceed 453 kg for taxi, takeoff, flight or landing.

Takeoff procedure

Before entering the departure runway the crew verify that it is the correct runway and entry point. On entering it the STROBE lights go ON, other lights as needed, and the transponder to TA/RA; when cleared for takeoff the FIXED LANDING lights go ON on most variants.

The PF advances the thrust levers to about 40 % N1, lets the engines stabilize, pushes a TO/GA switch and verifies that the correct takeoff thrust is set. With the flight directors on, TO/GA engages the takeoff modes, the pitch command showing 10° nose down until 60 kt and 15° nose up after. Any thrust adjustment is made before 60 kt, by hand if strong headwinds stop the levers short of the planned thrust. From 84 kt the autothrottle shows THR HLD. The captain keeps a hand on the thrust levers until V1 (see takeoff procedures and rejected takeoff).

After liftoff the PF follows the flight director, which keeps 15° nose up until the climb rate is sufficient and then commands the MCP speed, normally V2, plus 20 kt. Above 400 ft radio altitude the crew select a roll mode as needed and verify VNAV engaged; below 400 ft the takeoff mode can only be ended by switching both flight directors off. The autopilot may not be engaged below 400 ft AGL. In a normal takeoff the autothrottle stays in THR HLD until 800 ft above field elevation, then shows ARM. At the thrust reduction height the crew verify that climb thrust is set, and at the acceleration height the PF verifies acceleration and calls for the flaps on schedule.

On the 737 MAX, the FSB special training includes an erroneous high angle of attack during takeoff that leads to unreliable airspeed, continued into a go-around or missed approach, with special emphasis on the flight director bars biasing out of view when TO/GA is selected.

Flap retraction speed schedule

The flap retraction speed schedule retracts the flaps in steps: each new position is selected when the speed reaches the green flap maneuvering speed bug of the position just selected, shown on the speed tape. The bank angle is limited to 15° until V2 + 15.

The white V2 + 15 bug is removed at the first flap retraction, which also brings up the minimum maneuver speed bar (see B737 flight instruments and displays and B737 flaps and leading edge devices). On the MAX, retraction has another consequence: MCAS works only with the flaps up in manual flight, and the split vane monitor that compares the two angle of attack inputs is active only once the flaps are retracted.

After takeoff

When flap retraction is complete, the crew set or verify that the engine bleeds and packs are operating, set the engine start switches as needed and the AUTO BRAKE select switch to OFF. On the NG the landing gear lever goes to OFF once gear retraction is complete. The 737 MAX lever has only two positions, so there is no OFF to select; the FSB lists the changed After Takeoff checklist as a Level B difference and makes gear handle operation an item of special emphasis. The After Takeoff checklist completes the phase, and the climb and cruise procedure begins after it.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.