Wet Start (Boeing 737)

Wet Start – Identification, Causes, and Immediate Response

Applicable to Boeing 737-800 Series

Definition: What is a Wet Start?

A Wet Start refers to a condition during engine start-up in which, after N2 RPM reaches approximately 25% and the Engine Start Lever is moved to Idle Detent, there is no corresponding rise in Exhaust Gas Temperature (EGT). This indicates that fuel is entering the combustion chamber, but ignition has not occurred.

This situation can lead to the accumulation of unburned fuel within the engine, posing a significant fire hazard. If identified, the procedure must be aborted immediately.

Recognition: How to Detect a Wet Start

Standard operating procedures, such as those adopted by Boeing, help ensure a reliable integration between aircraft systems and pilot actions.

• During engine start, once N2 reaches 25%, the Captain moves the Engine Start Lever to Idle Detent and announces “Time Check.”

  
  

• The First Officer reads the second hand of the clock aloud, e.g., “One Eight” for 18 seconds, and monitors the passage of time closely.

  
  

• If the Captain calls out “Fuel Flow, EGT” within 10 seconds, ignition is assumed to be progressing normally.

  
  

• If 10 seconds elapse without a corresponding callout or EGT rise, the First Officer announces, “Ten Seconds,” indicating a Wet Start condition.

Immediate Response Procedure

• The Captain must immediately move the Engine Start Lever to Cutoff, terminating fuel flow and ignition signals.

  
  

• If the Engine Start Switch remains in the Ground position:

  
  

  • The Captain again calls “Time Check.”

    
    

  • The First Officer times 60 seconds during which the starter motor continues to rotate the engine, clearing any unburned fuel from the combustion chamber — a process known as Motoring.

If N2 exceeds 56%, the Engine Start Switch automatically moves to OFF. If N2 falls below 20%, the switch may be returned to GROUND to continue the motoring sequence as needed.

All actions up to this point are considered Memory Items.

Afterwards, the QRH Aborted Engine Start Checklist must be executed.

System Logic: Engine Start Lever and Ignition

On the Boeing 737-800:

• Moving the Engine Start Lever to Idle Detent opens the Fuel Shutoff Valves, activates the Electronic Engine Control (EEC), and initiates the Engine Start Sequence.

  
  

• Ignition is electronically controlled. There is no manual switch in the cockpit for independent ignition control; it is managed entirely through system logic.

Possible Causes of a Wet Start

1. Ignition System Failure

   
   

   • A malfunction in the ignition system prevents fuel from igniting.

     
     

   • If the Left Ignition system is suspected to be inoperative, setting the IGNITION selector to BOTH may allow a second attempt.

     
     

   • If the Right Ignition system is faulty, flight is not permitted as per MEL (Minimum Equipment List) limitations.

   
   

2. Tripped Circuit Breaker (CB)

   
   

   • A tripped CB may disable the ignition or fuel control systems.

     
     

   • CBs must not be reset without proper inspection by ground crew and maintenance personnel.

     
     

   • Before any reset, it must be confirmed that there is no fuel leakage or pooling fuel within or around the engine.

Resetting a CB without inspection may result in severe damage or fire hazard.