B737 flap extension over 20,000ft limit.

[SX737]

I would like some input on why Boeing has put a limit of not extending flaps above 20,000ft?? My first thought is that because of increased TAS if you increase lift by extending flaps at that altitude the stress imposed on the wings and flap surfaces can be catastrophic. Please feel free to input in this matter(and correct me!). Thanks

[hetfield]

This limitation is also valid for B744, A300, A320 family, A330/340. Just a certification limit?

[john_tullamarine]

Certification requirement .. there is no sensible operational need for flap at height so the OEM doesn't waste the time, effort and dollars on doing the substantiation ...

Covered in a couple of previous threads if you want to run a search for the details.

[Capt Claret]

Rather than thinking of it as a limit not to extend flap, think of it as JT has pointed out, not certified therefore one can't do it.

[Rivet gun]

Originally Posted by 737zaf

My first thought is that because of increased TAS if you increase lift by extending flaps at that altitude the stress imposed on the wings and flap surfaces can be catastrophic. Please feel free to input in this matter(and correct me!). Thanks

I think the stress on the flaps due to dynamic pressure would be slightly LESS at high altitude than at sea level. This is because Vfe is quoted in calibrated airspeed (CAS) and due to compressibility error EAS becomes less than CAS at high altitude.

Where you could run into a problem could be with Mach effects. Consider Vfe flap 1 = 250 knots CAS. At 20000 ft this is Mach .55 so no Mach problems.

However suppose you were to extend flaps at 250 knots CAS at 39000 ft. This is Mach .81 which is close to Mmo. You might induce unknown critical Mach effects.

[hetfield]

"Not extending the flaps at 20.000 feet" doesn't mean you may not fly at or above 20.000 if flaps are already/still extended.

[Old Smokey]

You'll eventually run into a Mach Number limit, that's why the 20000 foot limit was put there in the first place, as a simplified means of containing the speed within the Mach Number limit.

Some manufacturers do provide the Mach Number limit, but most simply avoid it by stating a maximum altitude for Flap operation where the Mach Number is below the limit.

What on earth do you want to operate the flaps above 20000 feet for anyway?

Regards,

Old Smokey

[hetfield]

Originally Posted by Old Smokey

You'll eventually run into a Mach Number limit, that's why the 20000 foot limit was put there in the first place, as a simplified means of containing the speed within the Mach Number limit.

Some manufacturers do provide the Mach Number limit, but most simply avoid it by stating a maximum altitude for Flap operation where the Mach Number is below the limit.

What on earth do you want to operate the flaps above 20000 feet for anyway?

Regards,

Old Smokey

Maybe not to "operate", but to have them extended.

Once we were on a short sector flight when flaps didn't retract after takeoff. The only available FL was FL 220......
We had enough fuel AND we didn't "operate" the flaps. So we made it to our destination.

Regards

[miaguppy]

Try This link.
http://www.smartcockpit.com/b737/B73..._EXT_LIMIT.PDF

[mutt]

This is taken from a Boeing Airliner magazine:

Several operators have asked Boeing why the Airplane Flight Manual has a limitation restricting the use of flaps above 20,000 feet. The reason for the limitation is simple; Boeing does not demonstrate or test (and therefore does not certify) airplanes for operations with flaps extended above 20,000 feet.

There is no Boeing procedure that requires the use of flaps above 20,000 feet. Since flaps are intended to be used during the takeoff and approach/land phases of flight, and since Boeing is not aware of any airports where operation would require the use of flaps above 20,000 feet, there is no need to certify the airplane in this configuration

Sometimes it pays to save old responses

Mutt.

[Old Smokey]

Extend, Extended, Retract, Operate.......ALL PROHIBITED above 20000 feet for the reasons given in excellent link provided by miaguppy. It's a Mach Number limitation.

The 20000 feet quoted is a convenient rounded figure, perhaps you had a lucky day hetfield, you may not have encountered a significant Mach Number until, say, 23000 feet?

Perhaps someone can retreive the details of the B727 crew back in the 70's, who inadvertantly (that's being kind, there was fiddling involved) deployed the leading edge devices at high altitude (Low CAS, but high Mach Number). The almost total loss of control and severe structural damage suffered in the extremely high speed descent almost lead to the total loss of aircraft and it's occupants.

Regards,

Old Smokey

[westhawk]

Who do you want to believe?

(that's being kind, there was fiddling involved)

Apologies for the slightly off-topic nature of this post, but I feel obligated to point out that there is legitimate reason to question the validity of the above quoted statement, in view of past and ongoing opposition to the original findings in that case. I am reluctant to accept this report as being purely factual. There exists sufficient doubt in my mind to justify inserting the word "alleged" between "was" and "fiddling" in the above statement. Just my view, but it appears to me that there was more evidence of system faults than there was of a crew conspiracy to fiddle with the flaps contrary to AFM limitations. Yet the conspiracy theory was accepted as the "official" cause.

The findings in that report (.PDF) of the TWA 841 727 upset were contested for many years by the crew and their union. Even TWA itself later joined in opposition to the original findings. I wasn't there or party to the original or subsequent non-NTSB investigations, so I cannot say with authority what really caused the upset. "Fiddling" was alleged in that case, yet there were several other cases of upsets on '27s where it was not. A very contentious case where the official findings remain in question to this day. The theory presented in the original report was contested on it's merits in THIS and THIS request to the NTSB for reconsideration of the probable cause finding. NTSB denied the requests. It's interesting to note that the board was split on the original finding and on the subsequent requests to re-open the investigation or re-consider the findings. Certainly an interesting read.

Anytime an airframe manufactures's interests stand in opposition to the interests of the flight crews involved following an accident, competing influences might tend to "muddy the waters" of investigative clarity and accuracy. With so much at stake, it's really no surprise that this might be so. One need look no further than the New York A-300 (AA 587?) or the whole B-737 rudder control issue to see what I mean.


Best regards,

Westhawk

[Old Smokey]

westhawk,

My apologies to all concerned in the TWA 841 727 incident, I have had access only to the NTSB report, and was unaware that there was any on-going resistance to, or disagreement with, the NTSB findings of the "Probable Cause".

In line with the thread topic, I believe it to be indisputable that the incident was initiated by the extension of Flaps / Slats at high Mach Numbers (Slats 2,3,6, and 7 and Flaps 2°), which then became far more serious when, upon Flap / Slat retraction, No. 7 Slat failed to retract.

Regards,

Old Smokey

[westhawk]

Old Smokey:

Thanks for your kind words. I have been interested in this incident and it's investigative outcome for many years. As you can no doubt deduce, I don't believe this case is an example of NTSB's best work when compared to hundreds of other reports I have reviewed.

I always look forward to reading your posts in the tech log forum. Asymetric or uncommanded flap/slat deployment at high altitude/mach number is indeed a serious matter which could result in grave consequences. I think I'll leave that sort of thing to the test pilots. My job is just to get from A to B in style!

Best regards,

Westhawk

[Jambo Buana]

Hi Old Smokey,

We had a RST session which involved a Flap stuck in position. Some crews wanted to take the problem home! This would have meant looking for altitude to get the range. We contacted Boeing who reconfirmed that you may not extend/retract or operate above FL200. This is due to mach effect. That is from the horses mouth, the Boeing Chief Tech pilot B737.

[hetfield]

The AOM for A300 says

Max. operating altitude for slat operation or flaps extended is 20 000 ft.

For flights with slats extended there is also a special operation chart in the performance section of the AOM with a preplanning graph up to FL 300.

Regards

[Old Smokey]

Thank you Jambo Buana, it's nice to be right once in a while.

Actually, hetfield's post reminded me of past Airbus days, where, indeed, Flap and Slat operation was not permitted above 20,000 feet, but, for a positioning flight with Slats or Kreuger extended, data was available up to 30000 feet, where, in addition to the CAS limit, a Mach Number limit now also applied.

Now I wonder why that might be ?

Regards,

Old Smokey