Boeing 737 Memory items – Engine Overheat

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 Overheat

Autothrottle (if engaged) ...................................Disengage

Thrust lever
(affected engine)...............Confirm..........................Close

If the ENG OVERHEAT light stays illuminated:
>> Go to the ENGINE FIRE or Engine Severe Damage or Separation () checklist

I found a very good explanation of the system in PPRuNe forum: https://www.pprune.org/archive/index.php/t-534700.html

Each engine has eight OVERHEAT/FIRE detectors, four in loop A and four in loop B. The detector consists of a 3-level pressure switch. Each detector is connected to an individual stainless steel, gas charged sense tube. 

The detectors monitor four sections of the engine; the upper and lower fan case and the left and right core section. 

In each section, two detectors (one of each loop) are mounted around the engine and make an assembly.

Gas in the sense tube is charged with a minimum pressure to keep the FAULT pressure switch closed. A leak in the sense tube causes the gas pressure to decrease and opens the FAULT pressure switch. Except for the OVHT/FIRE test, there is no flight deck indication of single loop failure. However the OVERHEAT/FIRE detection system will take the fault into account.

If both loops in one engine sense a fault in one of its detectors, the FAULT light on the flight deck illuminates and the system is inoperative.

A temperature rise inside and around the engine will cause the gas in the tube to expand. The increasing gas pressure will close the OVERHEAT pressure switch and further on the FIRE pressure switch.