737 NG flap load relief

[xetroV]

Hi all,

The 737 NG has a flap load relief system which retracts the flaps from 30 to 25 if the airspeed exceeds approx. 176 kts, or from 40 to 30 if the airspeed exceeds 163 kts.

Now suppose we have a blocked static port that causes the airspeed indication on the captain's or F/O's side to rise far into the red during descent. Will normal landing flaps (30 or 40) be available? The ALT disagree, Airspeed Unreliable and IAS Disagree procedures don't provide any clues.

Anyone able to shed some light?

Thanks! :)

[CaptainSandL]

The flap load relief function is controlled by the FSEU. This receives airspeed information from the left ADIRU. This gets static & pitot pressure inputs from the left air data modules (ADM).

So, if the Capts pitot probe and/or static port are u/s, the flap load relief function will not be available. This does not prevent you from using flaps but you will have to monitor airspeed from the F/O’s instruments.

Your scenario of extending the flaps “far into the red” is, I would hope, unlikely as you will have noticed the high speed from other clues - F/O’s insts, airflow noise, control feel etc. If you were to extend flap at high speed you would run the risk of damage or detachment which, if asymmetric, would cause large amounts of roll.

I have not got the link to hand as I post this, but search the internet for “N520UP AAIU Report No: 2004-004” which describes a 747 with static port problems on a post maint airtest.

S&L

[BOAC]

The simple answer is Flap15:)

[Contract Con]

xetroV,

I like a thinking man, well done!

Good food for thought...

I do like BOAC's answer, but can anyone answer xetroV's question from a tech point of view?

Capt SandL, i get what you are saying, however, why is it that the flaps will still run to 30/40 if the L ADM is supplying an IAS greater than the limit speed?

Normally if you select the flap above the load relief speed, it just stays at the previous setting until the speed drops below the load relief speed.

Cheers,

Con:ok:

[xetroV]

The flap load relief function is controlled by the FSEU. This receives airspeed information from the left ADIRU. This gets static & pitot pressure inputs from the left air data modules (ADM).

So, if the Capts pitot probe and/or static port are u/s, the flap load relief function will not be available. This does not prevent you from using flaps but you will have to monitor airspeed from the F/O’s instruments.
Well, consider an erroneous overspeed indication on the Captain's side, due to incorrect inputs to the left ADIRU. If the FSEU uses this incorrect airspeed, then I would imagine that the flap load relief system will prevent the flaps from moving beyond Flaps 25, even if in reality the flap placard speed is not exceeded. Unless the FSEU disables the flap load relief function in case of an IAS disagree. Does it?

Your scenario of extending the flaps “far into the red” is, I would hope, unlikely as you will have noticed the high speed from other clues - F/O’s insts, airflow noise, control feel etc. If you were to extend flap at high speed you would run the risk of damage or detachment which, if asymmetric, would cause large amounts of roll.
I see I wasn't very clear in my first post. The scenario I intended to describe does not involve an actual overspeed situation, just an erroneous overspeed indication on either the Captain's or the F/O's side. This could happen if a static line gets blocked during descent (sensed static pressure too low -> computed dynamic pressure too high -> computed IAS for the affected side too high, rising "far into the red" if you descent far enough). Of course, this would also cause an altitude disagree, with the indicated altitude on the affected side being 'trapped' as well.

I have not got the link to hand as I post this, but search the internet for “N520UP AAIU Report No: 2004-004” which describes a 747 with static port problems on a post maint airtest.
Thanks, this could be helpful!

The simple answer is Flap15 :)
Absolutely, but it is always nice to be prepared. :)

If the flap load relief system is inoperative in case of an IAS disagree, then there is no problem. But if it isn't, I'd rather know in advance if flaps 30 or 40 will not be available than to find out during final approach. After all, some runways are better suited than others when it comes to flap 15 landings.

[shadowfox64]

Hi,

In case of this error it should be possible to manual control the
flaps (overheadpanel) and select flaps as needed for the landing.

Regards John

[Brenoch]

Or just pull the c/b for the flap load relief system.

[TheGlide]

Hi thanks for the above thread I know it’s an old one but I have the same question but I’m looking for a reference if possible.

It would be much appreciated if anyone could help with these questions.

where does the FSEU recieve it’s airspeed information for flap load relief. Reference if possible I’ve checked everywhere possible FCOM MRG Bufer guide AFM

In the event of an airspeed unreliable will you get flap load relief from 30-25 if the airspeed is unreliably high.

I’ve read above that the airspeed info comes from the Left ADIRU but no reference.

Many thanks