B737 Normal Procedures: Preflight to Before Start

The Boeing 737 preflight and before start procedures are the normal procedures that take a crew from a powered airplane at the gate to the Before Start checklist. They bring every system to a known state, load the flight management computer with the flight, and confirm by inspection and test that the airplane is fit to fly.

This article follows the 737 NG flight crew operations manual (FCOM): chapter NP.11 for the philosophy and crew duties, NP.21 for the procedures. An operator's own standard operating procedures, built on Boeing's, govern its crews. Where the 737 MAX changes a preflight item, the article says so. Engine start and pushback follow in B737 engine start, taxi and takeoff procedures.

Normal procedures philosophy and crew duties

Boeing writes its normal procedures for a trained flight crew. They assume that all systems operate normally and that the automated features, LNAV, VNAV, autoland, autopilot and autothrottle, are fully used, without ruling out manual flight for proficiency. For each phase of flight they verify that the airplane's condition is satisfactory and that the flight deck configuration is correct. They also assume coordination with the ground crew before hydraulic systems are pressurized, flight controls are moved or the airplane moves.

The procedures are done by memory and scan flow, a set path around the panels shown in an FCOM illustration; its sequence may be changed as needed. Before engine start the crew read each system's condition or configuration from its lights; after start, the master caution system alerts them to warnings and cautions outside the normal field of view. A red warning or amber caution during or after engine start calls for the non-normal checklist and, on the ground, the dispatch deviation guide (DDG); an amber caution seen during recall that goes out after a master caution reset needs only the DDG.

Area of responsibility

Preflight and postflight duties are divided between the captain and the first officer, so each pilot's flow covers an area of responsibility. Phase-of-flight duties are divided differently, between the pilot flying (PF), who taxies, controls flight path and airspeed, configures the airplane and navigates, and the pilot monitoring (PM), who reads the checklists, communicates, does tasks the PF asks for and monitors. The mode control panel (MCP) is the PF's responsibility. CDU entries are made before taxi or when stopped where possible; during taxi the PM makes them and the PF verifies them before execution. The captain is the final authority for all tasks directed and done (see pilot flying and pilot monitoring).

Preliminary preflight procedure

The preliminary preflight procedure assumes that the Electrical Power Up supplementary procedure is complete (see B737 electrical system). An inertial reference unit computes position from the known position at which it was aligned, and its accuracy decays with time. A full IRS alignment is recommended before each flight; if time does not allow, the crew do the Fast Realignment supplementary procedure. The mode selectors go OFF, then NAV: the ON DC lights come on and go out, and the ALIGN lights come on. Until alignment is complete the CDU may show UNABLE REQD NAV PERF-RNP. A normal alignment takes 5 to 17 minutes depending on latitude, and is not attempted above 78°15' north or south (see inertial reference system alignment and modes).

The crew oxygen pressure drop test needs to be done at one crewmember or observer station only. Pressure must not fall by more than 100 psig; a rapid drop, a larger drop or a slow recovery suggests that the cylinder valve is not fully open (see crew oxygen systems). The fire extinguisher is checked and stowed, the crash ax and escape ropes stowed.

The procedure then sets or checks a series of panels: FLIGHT DECK ACCESS SYSTEM guard closed; ELT, PSEU and GPS lights out; SERVICE INTERPHONE OFF; REVERSER and ENGINE CONTROL lights out, EEC switches ON; PASSENGER OXYGEN guard closed and PASS OXY ON light out, since operating the switch drops the masks; FLIGHT RECORDER guard closed with its OFF light on. On the 737-8 the MAINT light replaces PSEU and REVERSER LIMITED replaces REVERSER.

Two tests follow. The MACH AIRSPEED WARNING TEST switches are pushed one at a time, and each must sound the clacker. The stall warning test needs the AC transfer buses powered for up to 4 minutes. With hydraulic power off, the leading edge flaps can droop enough to give an asymmetry signal and fail the test; the crew then obtain clearance, switch the system B electric pump ON, retract the flaps, repeat the test and switch the pump OFF again.

CDU preflight procedure

The CDU preflight procedure may start any time after the preliminary preflight, but it has two deadlines: initial data and navigation data before the flight instrument check of the preflight procedure, and performance data before the Before Start checklist. Every boxed item must be filled; the dashed or small-font items the procedure lists are entered or modified, and others are at the pilots' discretion. If the entered position is not within 4 NM of the origin airport, VERIFY POSITION appears. Leaving out the en route winds produces errors in flight plan time and fuel burn.

View full schematic

On the PERF INIT page the zero fuel weight must never be entered in the gross weight boxes, or the FMC computes performance with significant errors. The FUEL figure must agree with the dispatch papers and the fuel quantity indicators, and the gross weight and cruise CG with the papers. On the TAKEOFF REF page, page 2/2 is completed before page 1/2; it takes the thrust reduction height, 800 to 9,999 ft. If the FMC loses power for 10 seconds or more on the ground, every preflight entry must be made again. Page by page detail is in B737 FMC and CDU.

Exterior inspection

The exterior inspection follows a detailed route around the airplane. Along it the crew check that the gear struts are not fully compressed and that the skin around the pitot probes and static ports is not wrinkled; the 737 NG has three pitot probes, the auxiliary one on the first officer's side, and six flush static ports. Fluid leaks from the engine drains are allowed if they are less than a continuous stream. At the wheels, tires, brakes and wheels are checked and the wheel chocks verified in place as needed; with the parking brake set, the brake wear indicator pins must extend out of their guides.

On the MAX, the two-position tailskid, optional on the 737-8 and standard on the 737-9, adds a walkaround item; the draft Revision 21 of the FAA FSB report removes the tailskid, and its exterior check, from the 737-7.

First officer preflight procedure

The first officer preflight procedure starts on the overhead panel.

If the APU DET INOP light does not come on during the test, the APU must not be run. Done with the APU running, the test also flashes the fire light and sounds the horn on the APU ground control panel, which the ground crew can mistake for an APU fire. Two cautions apply to the fuel panel: the center tank pumps go ON only with more than 453 kg in the center tank, and never with the flight deck unattended.

Moving forward, the first officer sets the flight director switch ON, the PF's first, and the EFIS control panel to MAP with the weather radar off, and tests the disengage lights: in position 1 the A/P, A/T and FMC lights are steady amber, in position 2 A/P and A/T turn red while FMC stays amber. The flight instrument check allows only two flags, TCAS OFF and NO VSPD until the V speeds are selected; with the IRS aligned, the autothrottle, roll and pitch modes are blank and the AFDS status is FD. On the forward panel the AUTO BRAKE goes to RTO, the gear lever is DN with green lights on and red lights out, the ground proximity inhibit switches are guarded, and the engine display shows no exceedance and no RF on the hydraulic quantities. The start levers are at CUTOFF, the stabilizer trim cutout switches NORMAL, and the cargo fire test is needed once per flight day. The HF radio is never keyed during fueling.

On the 737-8 some of these items look different: its wing anti-ice valve alerts are amber where the NG's VALVE OPEN lights are blue, its four large displays come with revised display select panels, and its landing gear lever has two positions instead of UP, OFF and DN.

Captain preflight procedure

The captain preflight procedure includes the master LIGHTS TEST, which does not test the fire warning lights; lights that do not come on in this test are checked with their own test switches or push-to-test features, and the scan flow confirms that every other light is lit, including all system annunciator panel lights. The captain then sets the MCP: courses, flight director ON, the PF's first, bank angle as needed and the autopilot DISENGAGE bar UP. The STAB OUT OF TRIM light must be out and the NOSE WHEEL STEERING switch guard closed. On airplanes with a standby attitude indicator, its caging control is pulled and released and its approach mode selector set OFF, with no flags in view; the MAX has an integrated standby flight display instead.

On the control stand, the SPEED BRAKE lever goes to the DOWN detent with its ARMED, DO NOT ARM and SPEEDBRAKES EXTENDED lights out, the flap lever is set to agree with the actual flap position, the reverse thrust levers are down and the forward thrust levers closed. The parking brake is then set and its red warning light checked. The FCOM adds a warning: do not assume that the parking brake can prevent movement, because accumulator pressure can be insufficient. With systems A and B unpressurized, the brake accumulator, charged by system B, alone holds the parking brake.

Before start procedure

The before start procedure starts once the papers are on board. The flight deck door is closed and locked, with the LOCK FAIL light out. On the CDUs the PF normally displays the TAKEOFF REF page and the PM the LEGS page. On the MCP the autothrottle is armed, the IAS/MACH selector set to V2, LNAV armed as needed, VNAV armed, and the initial heading and altitude set. The pilot who will make the takeoff gives the taxi and takeoff briefings, which also rehearse the cabin altitude warning: if the intermittent horn sounds in flight above 10,000 ft MSL, the pilots don their masks with regulators at 100 %, establish communication and run the CABIN ALTITUDE WARNING or Rapid Depressurization checklist, removing the masks only once both have verified the cabin altitude stabilized at or below 10,000 ft.

The fuel and hydraulic panels come next. With more than 453 kg in the center tank, both center pumps go ON; their LOW PRESSURE lights come on briefly and go out, and a pump whose light stays on is switched off. If pushback is needed, the crew verify that the nose gear steering lockout pin, which depressurizes nose wheel steering, is installed. If the pin is not used, system A, which powers normal nose wheel steering, must not be pressurized, because unwanted tow bar movement can occur: its pumps stay OFF and only the system B electric pump is switched ON. Otherwise both electric pumps go ON. Either way, the brake pressure and each pressurized system must show at least 2,800 psi.

Then the ANTI COLLISION light goes ON and the trims are set: each checked for freedom of movement, the stabilizer trim set for takeoff within the green band, and the aileron and rudder trims at 0 units. A stabilizer outside the takeoff range would trigger the takeoff configuration warning once a thrust lever is advanced. The transponder is set as needed and the crew are ready for the Before Start checklist.

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.