When something goes wrong while flying on the Boeing 737, we use the QRH (Quick Reference Handbook). We just have to find out the correct NNC (Non Normal Checklist) and then we just read it and execute it.
For some of these checklists, we have memory items, meaning we have to know by heart some actions.
I have listed 9 memory items:
- Aborted Engine Start
- Airspeed Unreliable
- APU Fire
- Cabin Altitude Warning / Emergency Descent
- Engine Overheat
- Engine Fire, Severe Damage or Separation
- Engine Limit or Surge or Stall
- Loss of Thrust On Both Engines
- Runaway Stabilizer
Loss of Thrust on Both Engines
ENGINE START switches (both).......................................FLT Engine start levers (both)......................................CUTOFF When EGT decreases: Engine start levers (both)............................IDLE detent If EGT reaches 950°C or there is no increase in EGT within 30 seconds: Engine start lever (affected engine)................confirm..................CUTOFF, then IDLE detent If EGT again reaches 950°C or there is no increase in EGT within 30 seconds, repeat as needed.
Apart from an uncontrollable fire, the loss of thrust on both engines or double engine failure is probably the worst scenario a pilot could face. To cope with this kind of situation, even thought this is extremely rare, we need to understand what is associated with a double engine failure so we can react very quickly the day it comes and we don’t expect it (startle effect).
The goal of the checklist is to restart at least one engine but in case we can’t do that, the first thing to do is to turn the aircraft towards a potential airfield where we can land. At the same time, we do the memory items.
What happens when we lose both engine?
When we lose both engines, of course we lose thrust and as a consequence, altitude. After some time the IDGs (Integrated Drive Generators) will disconnect, meaning no more AC power.
The good reflex here is to start the APU. After it has started, we will get back AC power so electrical equipment back to normal and also EMDP (Electric Motor Driven Pump) but it supplies 6 times less fluid than EDP (Engine Driven Pump). However, the EDP continues to rotate as long as the engine is operating. I believe by operating we can suggest windmilling. How much pressure will be delivered will depend on the speed. More speed, more pressure and vice-versa. This is interesting to know for approach and landing as it will affect the time needed to put the gear down and flaps.
Here is an interesting answer I found on this website: https://aviation.stackexchange.com/questions/42565/does-a-boeing-737-800-have-a-ram-air-turbine-rat
- The B737 flight controls are hydraulically powered.
- There are three hydraulic systems: System A, System B, and Standby. Only one main system (A or B) is required for hydraulically flying the aircraft, during normal operation they are both operational.
- The two main hydraulic systems have an Engine Driven Pump (EDP), which can continue delivering hydraulic pressure when the associated engine is windmilling. All three hydraulic systems are also powered by their own Electric Motor Driven Pump (EMDP).
- In case of dual engine failure the APU can power the electrical systems for the EMDPs, still delivering full hydraulic power.
- If the fuel has run out and the APU cannot operate, two batteries provide at least 60 minutes of backup power for the electrical systems. The EDMPs can be powered in this stage, however they provide a high load.
- If all fuel is gone and the batteries are depleted, the aircraft can be flown by hand, directly overcoming the aeroforces by pulling hard! This is called manual reversion.
- In manual reversion, the aileron trim tabs now function as geared tabs, assisting in overcoming the aeroforces. Elevators will have high aeroforces, high friction forces, and freeplay around centre point. Stabiliser trim wheels provide additional pitch control. The rudder has no manual reversion.
Concerning the pressurization, if for exemple we lose both engines at 41000ft, the cabin is still pressurized cause the engines are windmilling. As we descend, the cabin pressurization needs to be maintained manually and as required. The first officer should be in charge of that while doing the checklist at the same time, so the workload would be very high in this kind of situation.
For a total loss of AC power and let’s say we can’t start the APU, that is what would be available or unavailable:
Note: Flight deck door will automatically unlock.
The 2 batteries provide a minimum of 60 minutes’ standby power. Each start attempt of the APU will decrease this time. (Approx 6min/attemp).
Below is a very good video demonstrating what was said above.