Learn the Boeing 737 engine start sequence: APU bleed setup, N2 fuel introduction, instrument checks and common simulator fault fixes.
To start a Boeing 737 engine, supply pneumatic air from the APU, a ground cart or another engine; configure the bleed-air system; select the engine start switch to GRD; then introduce fuel at the specified N2. Monitor EGT, oil pressure, N1 and N2 until the starter cuts out and idle stabilises.
For our Aviation & Real-World Flying readers, this explains the system logic rather than replacing an operator's approved checklist. Simulator users can follow the representative 737NG sequence below, but Classic, NG and MAX procedures—and individual add-ons—do differ. If the aircraft is completely unpowered, use our full cold-and-dark 737 simulator procedure first.
Boeing 737 engine start sequence
A 737 normally starts one engine at a time using bleed air from the auxiliary power unit.
- Establish electrical power. Configure the battery and standby power, then connect ground power or start the APU and place its generator on line. Electrical power operates the controls and instruments, but it does not itself crank the engine.
- Make the APU available. Wait until the APU is fully running before selecting its bleed-air supply. An APU that has electrical output but no bleed air will not turn the engine starter.
- Configure fuel and pneumatics. Select the required tank pumps on, set the ignition selector according to the checklist, turn both air-conditioning packs off, set the isolation valve to the appropriate
AUTOor open position, and select APU bleed on. Verify that duct pressure is available. - Obtain start clearance. In real operation, confirm the area is clear and coordinate with the ground crew or tug before illuminating the anti-collision beacon and rotating an engine.
- Select the engine start switch to
GRD. The start valve should open and N2 should begin increasing. Do not introduce fuel if N2 remains at zero. - Move the start lever to
IDLE. On a typical 737NG, this is done at 25% N2, or at maximum motoring speed when the checklist's stated minimum has been reached. The lever introduces fuel and enables ignition; adding fuel too early reduces cooling airflow and can cause a hot start. Our explanation of compressor rotation, combustion and jet-engine starting covers why this order matters. - Monitor the start. Look for fuel flow, an EGT rise, increasing N1, rising oil pressure and continuing N2 acceleration. The start valve should close and the start switch should return from
GRDautomatically. Many 737NG simulations model starter cut-out at roughly 56% N2, but that figure must not be assumed for every variant. - Start the second engine. Repeat the same sequence only after the first engine has reached stable idle.
- Complete the after-start flow. Place the engine generators on line as required, restore the packs and isolation valve, switch off APU bleed, shut down the APU when no longer needed, and configure probe heat, hydraulics, engine start switches and flight controls for taxi.
A printable PMDG NGX cockpit checklist is useful when flying that specific FSX add-on because it keeps the preparation, start and after-start actions in their proper order.
Which Boeing 737 engine starts first?
Engine 2, the right engine, is commonly started first, followed by engine 1, but the aeroplane does not require that order mechanically.
The operator's procedure, pushback method, airport restrictions, ground-air connection and maintenance requirements can change the sequence. In a simulator, either engine can normally be started first if the pneumatic and fuel systems are configured correctly.
What supplies air to the 737 starter?
The 737 uses a pneumatic air-turbine starter rather than a large electric motor attached directly to each engine.
| Air source | When it is used |
|---|---|
| APU bleed air | The normal source for independent starts at the gate or during pushback. |
| External pneumatic cart | Used when the APU is unavailable or APU operation is restricted. |
| Crossbleed air | Uses a running engine to start the other engine under an approved crossbleed-start procedure. |
Ground electrical power and ground pneumatic air are separate services. Seeing cockpit displays and lights does not prove that the starter has an air supply.
Why will a Boeing 737 engine not start in a simulator?
Most failed simulator starts come from missing bleed pressure, introducing fuel at the wrong time or overlooking an add-on's custom system logic.
- N2 stays at zero: check that the APU is running, APU bleed is on, the packs are off, the isolation valve permits airflow and the start switch is in
GRD. - N2 rises but there is no light-off: check the engine start lever, fuel pumps, tank quantity and ignition configuration. Fuel is not introduced while the start lever remains at
CUTOFF. - EGT rises rapidly while N2 stagnates: this indicates a hot or hung start. Move the affected start lever to cutoff and follow the aircraft's abnormal-start checklist rather than repeatedly adding fuel.
- The engine starts but electrical warnings remain: complete the after-start flow and place the engine-driven generators on line if the model requires manual selection.
- The simulator's automatic-start command fails: study-level 737 add-ons often replace default simulator logic with custom electrical, fuel and pneumatic systems. Use the add-on's own checklist or panel controls.
A healthy start has a clear order: pneumatic rotation first, fuel and ignition second, then stable acceleration to idle. Starting the APU without opening its bleed valve—or leaving both packs consuming pneumatic air—is the mistake we see most often.