737NG Ground spoiler system

[BOAC]

Following another thread, and some very useful info (thanks 'mono') it appears that failure of the GS Interlock would cause an intermittent horn when airborne.

I need a little help here please, techies (and CaptS&L, of course) - as this component is undocumented in 'Pilot's Notes' and MEL/DDM - with this:-

1) I guess the horn would sound at lift-off, thereby being a pretty good clue that you do not have a 'cabin altitude' problem

2) Could any sort of failure later cause the horn to sound?

3) I assume that it cannot be silenced until next landing unless the CB is pulled?

4) What, if anything, would determine a call for GS extension in the air with the failure?

5) I am assuming that the PSEU also controls GS deployment?

6) Presumably therefore there would be no adverse effect on roll control in a failure mode?

[CaptainSandL]

Hi BOAC, I will have a go.

The AMM (31-53-00) states “The takeoff warning function gives an aural warning sound if the airplane is in an unsafe condition during takeoff or if the ground spoiler interlock valve remains open after takeoff.”

To clarify this, excluding cabin altitude warnings, the intermittent horn (I wont call it the T/O config warning horn because it isn’t always that) will only sound on the ground if the thrust levers are more than 53 degrees AND
• Speedbrake lever is not down
• Parking brake is set
• Ground spoilers have pressure
• LE flaps and slats are not extended, or have an uncommanded motion
• TE flaps are not in a takeoff position, or are in a skew or asymmetry condition, or have an uncommanded motion
• Stabilizer is out of the green band.

Or in the air when the landing gear takeoff warning cutoff circuit breaker is closed, the aural warning system gives a takeoff warning sound if both of these conditions occur:
• LE flaps and slats are not extended
• Ground spoiler interlock valve is open.


To answer your questions specifically:
A1: Yes, as soon as the LED’s have retracted.

A2: I don’t know, but I don’t think so.

A3: Correct, it will sound until the first flap selection. Maybe keep flaps at 1 or more if you get this problem to help you think?

A4: Now sure exactly what you mean, but when airtesting we switch the SPOILER A & B switches off to ensure that no panels deploy when not locked down. This is done by the CAA schedule at FL350 / M0.74 / Clean and by the Boeing schedule at <FL200 / 130kts / Flap 40. Both configs are designed to generate a large pressure differential on the wing which would show any tendency to lift a panel. The reason for this test is because of an incident to the DC9 which was nearly lost (http://www.kafena.net/accident-reports/look.php?report_key=1069)when the crew selected full flap on final and the aircraft rolled because a spoiler panel became unlocked and deployed.

A5: Not sure, the finer points of the PSEU are lost on me; but the PSEU certainly commands the aural warning module to sound the intermittent horn.

A6: NO!!! this has a potentially big effect, see A4. That said, I have done approx 200 spoiler float checks on 737’s with no roll control problem or roll observed.

S&L

[BOAC]

Thanks for the reply - I've left this with our eng's while I zip up to KEF. if both of these conditions occur - that's useful to know

Your points:

1) :ok:
2) I was musing whether a PSEU WOW malfuntion would trigger? As you say, a bit of a 'black magic' box, the PSEU
3) :ok:
4) I was thinking of g/s deployment on selection of speedbrake with the malfunction.
5) Our eng's have confirmed the g/s interlock signal (driven by a Telix cable on stbd gear) routes via the PSEU
6) I was asking whether g/s panels could extend hydaulically rather than float with this failure - see 4).

[CaptainSandL]

Re g/s deployment on selection of speedbrake with the malfunction. I am not sure but I would not dare operate the speedbrake in this condition in case all the panels came up. It is certainly a risk with any air/ground logic fault that using speedbrake may cause the ground spoilers to deploy.

[737 Checker]

Yes, Ground spoilers could also deploy. If I remember correctly the gear lever will not go up without pulling the trigger if the interlock cable is in the on ground position.

[BOAC]

the gear lever will not go up - please confirm you refer to NG and the interlock failure and not just a/g switching?

[mono]

In a way I started this so I may as well add my tuppence.

The intermittent horn sounds for a ground spoiler fault under the following conditions (ON THE 737NG)

A/C in the air (PSEU function)

Leading edge devices retracted (FSEU function then fed to the PSEU)

Ground spoiler interlock valve open (prox sensor fed to the PSEU - there is also a pressure switch but this is not used for in air intermittent horn)

Landing gear take-off warning cutoff CB closed (on P6)

Regarding the interlock valve - This is a mechanically operated valve, operated by a teleflex cable attached to the lower (sliding portion) of the right main landing gear. So, if for some reason the gear does not extend OR the valve does not close (cable seized, valve seized, etc) then the valve will still be open (note the valve is fail safe and is spring-loaded closed so if the cable breaks it should close)

If the a/c is in the air and the spoiler interlock valve is open the horn will sound as soon as the last LE LED transit light goes out and the LE devices are retracted.

To silence the horn the landing gear take-off warning cutoff CB (on P6) can be pulled. This will cancel the take-off warning horn but will still allow the cabin pressure warning to sound.

OR the landing gear aural warn CB (also on P6) can be pulled. This will silence the horn but will ALSO prevent a cabin pressurisation warning should it occur.

The PSEU has 2 systems 1 and 2, only system 1 is used in the control logic for the horn to sound. The logic within the PSEU seems to be fail safe (as it should be) in design. logic gates requiring voltage (rather than earths) to go true and the final electronic switch is powered (rather than earthed or logic low) to sound the horn. Therefore I doubt a PSEU failure would cause an erronous horn (more likely it wouldn't sound when it should!). BUT - the input to the aural warning unit from the PSEU for the horn to sound is an earth, so a chaff to ground downstream of the PSEU COULD cause a false warning.

Note also that if the gear fails to "unsquat" a secondary effect would be to prevent the normal retraction of the gear. Perhaps the reason for 737 checkers' response.

All gleaned from ASM 31-53-11 'cos the AMM these days is s**te

Hope this helps.

Cheers.

[BOAC]

Company eng's have now provided a diagram showing the 'flow' - thanks all for the 'heads up'.