View Full Version : Flap load relief on 738, big deal?

9th Oct 2007, 14:23

this is what happened. Vectors for an ILS approach, we were cleared for intercept and were still 1500` high(Controller didn`t let us down). Anyway it ended up being 2 dots above glide, we switched off the autopilot and flew manually to catch the glide from above. Extended the flaps to 30, pitched quite heavily down...Then I saw for just a short time the flap load relief light. I can`t remember the speed we had at this point but we ended up stabilized nicely at 1000`AGL with an uneventful landing.

My question, I know this should never happen, but is this now a big deal?
Do we have to fear any consequences??
Thanks for any comments.


9th Oct 2007, 14:27
It is a flap exceedance and should be inspected...but no, it's no biggie...

9th Oct 2007, 14:44
hello wk,

not a big deal at all. flap load relief is a protection system which apparently worked as mr. boeing advertised.
just 2 things to do : 1) make a tech log entry for maintenance. 2) submit an sair (safety alert initial report), indicating what happened. normally that's all there is to do & nothing more to be heard of. just a word of caution, if this would happen within the "landing gate"(below 1000 ft agl/imc or 500 ft agl/vmc), a standard go around( cause aircraft is not stabilized) is mandatory as per sop's.
kind regards,

9th Oct 2007, 16:22
flap load relief light

Is this an extra? Never heard of it before. Also, did the flaps move up, ie. 40 to 30 or 30 to 25?

Jet Man
9th Oct 2007, 18:31
Some NGs have a flap load relief light that lights when flap 30 (800s) or flap 40 speeds are exceeded - it indicates flap 'blowback'.
In this case I wouldn't have thought that a tech log entry was needed unless the flap 25 speed was exceeded.

9th Oct 2007, 18:43
hello everyone,

b737-800 have the blown back feature for flaps40 & flaps 30. the caution light is optional.
in both cases a techlog entry is definitely required, as it will show on the "ofdm"(operational data monitoring system)= big brother is watching you.
kind regards,

10th Oct 2007, 12:54
Why would a tech log entry be required when a system (not part of any emergency/backup system) worked as advertised? I've personally seen lots of flap load relief events in B747 classic and B744 over the years. It's not a big deal, it's a normal part of the flap system as certified.

I am, of course, assuming that the B737NG is the same as the B747 in this area.

10th Oct 2007, 13:14
See http://www.b737.org.uk/flightcontrols.htm

The flaps will extend again when speed is reduced. This feature is on all (737) aircraft even though the FLAP LOAD RELIEF light is only fitted to a few.

So, in some aircraft and with a 'quickie', you may not even know about it. How would that need a Tech Log entry?

10th Oct 2007, 15:10
hello everyone,

if the flap load relief system works as advertized, it is still a flap speed exceedance as said in another post & although no harm is done it nevertheless needs a techlog entry & precautiony flap inspection as per maintenance manual.
it is part of the safety culture. to the attention of forget:
even if no light is fitted, it is not that "quicky" . you will feel/notice the aerodynamic configuration changes(even on auto pilot) & your pm(pilot monitoring) should advise about flap placard speeds & monitor the flap gage.


10th Oct 2007, 15:26
blackmail, Here back :ugh: ...... you will feel/notice the aerodynamic configuration changes(even on auto pilot) & your pm(pilot monitoring) should advise about flap placard speeds & monitor the flap gage.

You're a better man than most Gunga Din. Let's say you select Flaps 40 and the flaps are motoring past 30. Momentarily you exceed the Flap 40 limit. Flap Load Relief kicks in, momentarily, and reverses flap motoring, momentarily.

You'd notice that would you?

Here, have another one. :ugh:

10th Oct 2007, 18:13
forget, hello,

placard flap speed for flap 30 = 175kias & for flap 40 = 162kias ( b738ng)

boeing manual says: when flaps are set at 40, the TE flaps:
-retract to 30 if speed exceeds 163kias & re-extend when speed is reduced below 158kias. when flaps are set at 30, the TE flaps:
-retract to 25 if speed exceeds 176kias & re-extend when speed is reduced below 171kias. so in both cases a difference of 5 kts exist between retraction/extension, which may seem small. but when exceeding the placard speed the aircraft is still probably accelerating a bit & then to decelerate again about 5 knots, definitely requires pilot intervention & will take more than "momentarily" to achieve, so you will definitely take notice. those flaps just don't retract/extend as windshield wipers.
kind regards,

Jet Man
11th Oct 2007, 00:16
I have only flown aircraft without the flapload relief light. I have seen the flap blowback but only in moderate+ turbulence on approach - I never 'felt' it blowback!

None of the three operators I have flown the B737NG with have required tech log entries for flap blowback as this is a normal aircraft function. Other operators might differ of course.

11th Oct 2007, 06:36
Boeing in the 747 maint manual states that the flap load relief will prevent excessive airloads on the flaps by retracting T/E flaps from 30 to 25 & 25 to 20 if it approaches within 2 kts of the placard limit. As a LAME this does not initiate an inspection unless it is reported in the log that a flap placard SPD was exceeded & by how much, in this case 1 & 15 kts are the 2 criteria of interest, (Phase I or Phase II inspection required). It would be up to the pilots to monitor IAS Vs Flap Ref. and report if required.
Mr Boeing does not like change so I can assume the 737-800s will be very similar if installed.

11th Oct 2007, 06:50
It seems that there IS a difference between the B737 and B747 when posts #11 and #13 are read.

A tech log entry is definitely required if a limit speed is exceeded, even by 1 kt, but no such entry is needed unless and until there is an exceedence.

If you happen to be caught out high and fast, selecting ldg flap usually does no good because of approach (high) idle selected with ldg flap. You will usually descend quicker with gear down, F20 and speed CAC.

11th Oct 2007, 07:15
Yes, Gander is correct, I did not RTFQ, Bm's figures show the 737-800 must be in exceedance by 1 kts to initiate a load relief, if the info is true it must be written up in the log for maint action.

WK, no its not a big deal as the inspection could be put off for a few flights or an ONS if only small O/SPD. (again ref 744).

11th Oct 2007, 11:18

We just received a brand new 737-800 and there is a flap load relief system operating not only at flap 30/40 but also on 25/15/10

model is YK910



11th Oct 2007, 19:32
So a guy asks a technical question about his airplane. If a limit is exceeded, it should be written in the techlog. But if the flaps retracted automatically because the system sensed a speed too high for that selection, or didn't even extend when the selection occured at a way-too-high speed, - was there ever an overspeed situation?
Now if they did NOT retract automatically when they should have, that's what i would see as an exceedance of the flap limit. And have it written up. But I quite wonder why some folks here get personal, or wise with SOPs - it was just a technical question, wasn't it?

Happy landings

12th Oct 2007, 13:41
it appears there is a dif between maint manuals and airline ops. exceed the limits and report is always law, but in this case, a flash of a light may not be an indication of exceded limits. depends on the A/C you pilot and airline that pays you. I say, if the light illuminates, report it...