B727 flap use above FL200
 

Air crash investigation last night…

B727, a non approved procedure of extending trailing edge flaps to 2 whilst pulling the leading edge breaker to prevent any slat movement at FL390…?

How on Earth does this give better ground speed as suggested in the program. Maybe delays any Mach tuck?

penny for your thoughts

I think they were trying to get more lift out of the wing whilst climbing to a level that really was too high for them to get out of the strong headwinds and thus improve their groundspeed. These drama/documentaries tend to be a big "light" on technical details!

Increased surface area of the wing allowed a “alternate” (higher) Optimum Altitude therefore better winds hence a better ground speed.

Thanks. Line pilots becoming unqualified test pilots on revenue flights to save a few mins.

Well nobody flies 72’s anymore on pax revenue flights.

There's an interesting paragraph or two in "Handling the Big Jets" which explains the highly arbitrary nature of the FL200 limitation. And also why the manoeuvre margin changes at that level.

I’m no expert but what about the drag? Must be noticeable at high speed..

With Boeing types the initial degrees of leading edge devices promotes more lift than drag. The well documented 727 incident which nearly ended in catastrophe is more explainable by human factors and stupidity.

Critical mach
 

The trident had a droop limitation of fl230 iirc as the flow would become locally supersonic, ignoring it led an involuntary descent through the Clacton hold before the crew could recover the aircraft.
There were a couple of captains that would descend a few hundred feet at cruise level to accelerate the aircraft and allow it to fly with a lower nose attitude would reduce parasite drag and allow a higher cruise or lower fuel consumption.That's the theory they spouted..there is also the ram air effect to take into consideration.

This wasn't the first time it had been done but probably the last.

Getting the aircraft "on the step" was a feature espoused by many old timers flying pistons, Ernest Gann among them. The theory, as I understand it, relates to the drag curve, there are two speeds at which an aircraft can fly with a given amount of thrust, could possibly see the technique working in a thrust limited situation.

Hi Anson...which version of HTBJ are you referring to? I can't find it

It's been 15+ years since I read it, but wasn't it simply that Boeing decided they had enough data and didn't feel like going above FL200 was necessary?

Never flew the B727, but on the Boeings that I did fly there was a limit of FL200 for flap extension. I believe the reason was simply that no relevant flight tests had been done. This ties in with #13 above.

787 has cruise/climb flaps. The inner and outer flaps are physically disconnected by a clutch mechanism, allowing the inner flaps to extend by a few inches to increase performance.

3rd edition Dec 71 page 170 refers to the use of flaps at high altitude. DPD refers to pilots who have developed a habit of using flaps at altitude which has raised concerns. His primary concern is that flap design is based on their use for TO, climb, decent and landing and will be retracted for the rest of the flight. The strength and fatigue calculations for the flaps are based on this normal use. He goes on to point out that the flight manual limitations for flap use are there for reasons and should be observed.

I used to fly with an ex BOAC VC10 captain who said there was a trick to improve the climb by splitting the flap/slat lever and doing something to either flaps or slats. When the Trident crashed at Staines the practice stopped overnight.

​​​​​​Use of flaps at high altitude

It would appear that in recent years pilots have developed the habit of using

their flaps at high altitude, presumably for holding or descent purposes.

This rather indiscriminate use of flaps has caused some concern. While the

captain of an aeroplane can of course take any action he considers justifiable

under a special set of operational conditions, there are good reasons for

not using flaps in other than the normal circumstances.

Firstly, flap design is based on the assumption that the flaps will be used for

take-off and climb, and approach and landing, and that they will be retracted

for the rest of the flight. On this assumption are based the strength and

fatigue calculations of the flaps. If they are used at other times then the

original design assumptions are compromised. Secondly, there is a lower

level of proof strength with the flaps extended; this is quite often + lg

increment instead of the + 1 ½g increment in the flaps retracted case. Lastly,

on some of those types fitted with stick shakers and stick pushers, the

design of these systems is such that, with the flaps extended above certain

altitudes, the shake and the push will not function in the correct fashion and

the aeroplane will be denied some of its normal stall protection (in the clean

configuration stall protection is provided at very high altitude by the marked

pre-stall buffet due to Mach number effect, regardless of the point at which

the stick shaker operates).

In this last instance specific limitations on the use of flaps above certain

altitudes will be found in the Flight Manual. They are there for good

reasons, so observe them. Unless there are specific operational reasons the

use of flaps should be confined to the normal low altitude fun􀁊tion. While

you are about it, do try to stay within the speed limitations; flight recorder

results show far too many occasions on which the flap speeds have been

exceeded, in some cases by large margins.

dixi188, I would like to know how your ex-VC10 captain knew that his special method gave him any advantage. Without doing repeated runs (or climbs) under controlled conditions with and without his special technique, and measuring the difference, there would have been no way he would have known. That is why the industry has test pilots. Does your average line pilot know how to measure performance climbs, etc?

Thank you Extreme P and Happy, found it!