View Full Version : Fuel Filter Bypass

29th Oct 2005, 16:58
Recently we've had a high speed abort because of the FILTER BYPASS light illuminating during T/O. (734)

Of course the Captain was called by the boss for a coffee no biscuits meeting.

Besides that, your thoughts would be appreciated on the following:

is it right to say that if the fuel is really contaminated, the first thing to be clogged will be the booster pumps filter?

That illumination of the Filter Bypass light alone is virtually impossible?

We've had several false cautions last year with this problem, and it always was a faulty indication.

How the dirt in the fuel could block the filter in the MEC without accumulating first at the booster pumps filters?

gas path
29th Oct 2005, 19:46
First a disclaimer: I do not know the 734 or the CFM56!
(BUT) If the 734 fuel system is similar to the rest of the Boeings the boost pump pick up 'filter' would be nothing more than a VERY coarse strainer so it would be unlikely to clog*, and possibly (if it did) the first hint would be a low pressure light/warning from the affected pump itself.
There are a number of reasons for FILTER BYPASS and if no fault is found with the filter itself (it may need tearing apart to see if it is contaminated)another possibility could be ice forming in the filter from cold fuel. A good case for thorough water drains checks!
Next stage would be changing the diff. pressure s/w. and more remotely a wiring fault/chafe somewhere!
I would hope that the filters are replaced on a regular basis.
As for the high speed abort, probably the wrong call but it would depend on your procedures.

*It is designed to stop the odd fitter/poly bag/carpet/rag/nut/bolt/washer from getting as far as the pump impellor!:uhoh:

1st Nov 2005, 10:48
I'm not familiar with the EICAS on a 734, but presumably a caption like this would be accompanied by a master caution? Or would the M/C be inhibited...? If you get an M/C before V1, surely there is no case to answer ... or do some outfits only think you should reject for the big red ones?

Gary Lager
1st Nov 2005, 11:09
1. Correct

2. No Caution inhibits on a 734.

3. RTO due to 'M/C below V1' - our company SOPs advise once thrust set/~80KIAS, rejection solely due to Master Caution is rarely necessary; only for Eng Fire/Fail/other condition which affects the ability of the aircraft to fly safely. So a M/C below V1 is not an automatic RTO, but decision is left to the crew bearing in mind the safety implications of rejecting at high speed.

1st Nov 2005, 14:51
I might also be influenced by the last fuel uplift - if it were from a site with a history of problems.... :suspect:

Krystal n chips
2nd Nov 2005, 17:28
Gas path offers a wide range of potential causal factors, all of which are wholly relevant, but also hits the nail firmly on the head with the two prime causes--filter change--or lack of to be more precise--and ice due to water. Water drains are there for a purpose and I admit that, whilst I did not enjoy the task I always did it thoroughly---and with all due respect to aircrew here--you would be amazed just how much water can get drained at times.

As I recall, Quantas always insisted a sample be shown to the Flt Eng as part of his walk round--which shows how much emphasis they placed on this particular and avoidable problem.