FEEL DIFF PRESS light ON just after lift-off

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FEEL DIFF PRESS light ON just after lift-off

Postby maro12 » 30 Aug 2019, 21:53

Good Day Everyone,

737-700 (with FEEL SHIFT MODULE)

After Elevator Feel Computer overhaul experiencing following problem: FEEL DIFF PRESS light comes on just after aircraft lifts off, stays steady during flight, goes off upon touchdown.

Pilots did not report any anomalies with handling the aircraft (3 flights in a row) although during last flight reported "BANK ANGLE" annunciation after takeoff with AP engaged (disconnected AP and continued without further anomalies except FEEL DIFF LT). Pilots hyd press observation: no press difference between A and B systems.

Weather condition: hot, sunny day, clear sky (around 30deg C)

Troubleshooting steps performed (following the FIM 27-31 TASK 815):

->Exercised R00813 30-seconds relay delay circuit (switching off FLT CONT switches, one at a time - LT ON within time limit, with LT ON opened Right Elev Tab CB - LT OFF, closed CB - LT ON after 30 sec, both hydraulics OFF - LT OFF, both hydraulics ON - LT OFF), exercise carried out several times, satis
->Flushed Elev Pitot and static lines and cleaned restrictor valves, leak test satis,
->All hyd filters have been replaced
->FDR readout - no hyd press difference during affected flights
->bleeding of elev hyd system performed (with application of 300 kts with air data test set to both elev Pitot probes) light did not come on during simulation - I also put ac in air mode with 300 kts on elev Pitots - Light did not came on...
->feel force test performed - satis
->Neutral Shift Module test performed - test ok

I did not do check of resistance of Pitot probes as I assumed no possibility of icing (LT on just after lift-off during hot clear day) - please correct me if my assumption is wrong.

I did not inspect rubber flex lines, although I tried to blow air with my mouth from the Pitot lines in Stab bay (with restrictor valves removed and lines capped accordingly) I could feel air flow - I would feel if line would be kinked or blocked.

I can not see any interface with air-ground system. But strangely LT comes on immediately after takeoff, goes off after touchdown.

All tests on ground passing without reproducing the fault. Once ac lifts off - DIFF PRESS LT ON.

We are going to replace FEEL COMPUTER as a next step.

Do any of you guys have experience with similar snag?

Thank you in advance for any ideas and feedback.

Best Regards,
Posts: 117
Joined: 05 Oct 2008, 01:51

Re: FEEL DIFF PRESS light ON just after lift-off

Postby maro12 » 02 Sep 2019, 18:54

Good Day All,

Quick update: Elevator Feel Computer replaced and tested satis. FEEL DIFF PRESS light did not come on during flight. Fault cleared.

Regarding "BANK ANGLE" aural annunciation: DFCS BITE including Fault History check performed: found Elev Pos Snsr and Neutral Shift Snsr faults. Checked sensors adjustment and found those two sensors slightly off (on the limit I would say, even BITE once passing, later on during the day failing). Sensors adjusted, all tests including Land Verify satis.

I am not sure if and how these two pitch channel sensors could be the root cause of reported instance of exceeding bank angle limit but we did not find other DFCS issues.

Subsequent flight with no faults.

Best Regards,
Posts: 117
Joined: 05 Oct 2008, 01:51

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