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
Engine Limit or Surge or Stall
Autothrottle (if engaged)....................................Disengage Thrust lever (affected engine)..................Confirm................Retard until engine indications stay within limits or the thrust lever is closed
First, it is good to have a look at this video to review how a jet engine works. Here is the CFM56-7B, used on the Boeing 737-800.
What is an engine (compressor) stall?
All the blades attached to the compressor are similar to a wing airfoil. Like a wing, it is possible for the blades to stall. If the airflow gets disturbed, whatever the reason (bird ingestion most likely), it might cause a high angle of attack over the blades until it stall and a reversal of the airflow. When the compressor stalls, the airflow is no longer flowing normally through the combustion chamber and the turbines. The air, used for the combustion with the fuel helps also a lot for cooling. As the airflow is blocked, there is no longer any cooling and the temperature is massively high in the combustion chamber area, that’s the reason why we will get a sudden high EGT on the indications.
Now, basically we have 2 options and these 2 options make the difference between a stall and a surge.
For a stall, the pressure and airflow at the last stage of the compressor decreases significantly. There is not enough air to burn the mixture. The hot unburned fuel goes through the back of the engine where it meets with the air coming from the fan (high bypass), this causes a sudden combustion and that’s why we see flames coming out of the engine.
What is an engine (compressor) surge?
For a surge, the pressure at the compressor outlet increases suddenly with a slight reduction of the airflow. This creates the perfect conditions for burning the present fuel but not enough air for cooling the whole. This causes explosions which can either escape by the front or the back of the engine. A surge is more dangerous than a stall in the sens that it’s kind of self-sustaining until the pressure equalizes. It creates loud bangs and big vibrations. If nothing is done, this can lead to engine severe damage.
Below is the best video I found explaining the process:
What could cause a compressor stall/surge:
- Bird strike
- Foreign Object Debris (FOD). We understand more the importance of visual runway inspection.
- Abrupt movement of thrust lever
- Inflight icing. Let’s say you accumulate ice on the engine cowling, when it breaks off, it could damage the fan blades resulting in compressor stall. Or when engine anti-ice is used, run-back water can refreeze on unprotected surfaces of the inlet and, if excessive, reduce airflow into the engine or distort the airflow pattern in such a manner as to cause compressor or fan blades to vibrate, possibly damaging the engine. (see http://www.aviationchief.com/icing.html#)
To summarize, wrong compressor angle of attack and too rich fuel air ratio are the main causes of compressor stall.
Modern jet engines use several things to counteract this problem.
- Variable inlet guide vanes. Located just behind the fan. By adjusting these vanes, we optimize the angle of attack of the rotor blades (compressor).
- Bleed valves. These valves open allowing some airflow and reducing the compressor stall effect.
- Limiting the fuel supply thanks to the EEC (Electronic Engine Control).