CFM56-7B Engine fuel leaks during engine start

Many times 737NG’s are delayed due to engine fuel leaks found underneath the engines.

Especially when the engines are started, a large amount of fuel can be drained overboard , coming from under the engines. This alarms the ramp personnel and this may cause questions and delays. Often this happens when a cold engine is started on the first flight of the day.


The main suspects are the HMU, HPTACC valve and fuel pump, which have o-rings and carbon seals installed. The o-rings can shrink a little when they are cold and expand when they become warm again. This may cause a small amount of fuel leakage. Also the carbon seals may leak fuel when they are not forming a closed seal when not rotating. This all together may cause the fuel coming out of the drains when starting a cold engine. Also on a non-rotating engine, after a while, the engine may leak fuel out of the drains. This happens during heavy maintenance where the engine is not operating for many days/weeks.


Many times the leaks are well within amm limits, if checked after the 5 minutes idle run, as described in this article on this website.


To help the flight and ground crew in this situation, the FCOM is updated by Boeing in were this fuel leakage is mentioned, and they recommend to keep the engine running at idle for 5 minutes. When after 5 minutes the leakage stops, no action is required. When the leak continues, then the engine must be stopped and checked.


So remember: DO NOT PANIC or ground the a/c immediately, but first do the ground run for 5 minutes and then see if it still leaks.


Ref FTD 73-15001 and AMM 71-71-00-200-801-F00 2 (A) (2) and AMM AMM 71-71-00-200-801-F00 2 (E) (2)


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