Flap Limiting Speeds - IAS vs TAS

[The Beakster]

In my aircraft type, it is common to configure fully at cruise altitude (up to FL280) and then fly a continuous configured descent at 5 kts below the full flap limiting speed.

I’ve been concerned for some time that in doing so, your TAS is up to 90 Kts above the flap limiting speed. I’ve done extensive research in both my aircraft documentation and online, and have so far established the following:

• Flap / Gear limiting speeds are initially established as a TAS, and then factored and converted to IAS for ease of pilot use.

• On some aircraft, a max flap selection altitude is specified (presumably to protect against such a scenario?).

• On occasion, aircraft have overstressed in this way. One extreme example was the advertent selection of gear at 300 KIAS in an SR-71, which was actually travelling at Mach 3.02.

• My aircraft docs do not indicate any restriction on the altitude at which services may be selected. However, until recently it would have been VERY unusual to configure at FL280 (other than an emergency descent). This would normally have occurred at <3000 AGL.

Whilst I appreciate that flap limiting speeds will contain a factor of conservatism, it seems strange that a buffer of 90 kts has been used. This would effectively mean that full flap could be selected at just 35 kts below the highest Vno at 1000ft.

I summary, I suspect / am concerned that we are overstressing or exceeding the flap limiting speed. However, I do not have a background in T&E or any technical knowledge beyond ATPL level. I would really appreciate any thoughts or explanations available. If you need more specific info, please PM me.

Many thanks in anticipation of your help!

[FE Hoppy]

Dynamic pressure!

[BOAC]

A little confused as to where you see a 'buffer' of 90kts and again where the '35kts' comes from?

However, flap limits are based on IAS which as FEH says is to do with dynamic pressure and load on the surfaces. I have never heard of a TAS limit. The 737 has a limit of 20,000ft for flap extension, thought to be for two reasons
1) No-one has 'tested' it higher
2) Compressibility effects start to come into play at that altitude and changing wing camber in that area could have totally unexpected consequences.

As for descending from 280 5kts below full flap limit.........................I hate to think what that does to your airframe life! Is it that hard to get it down?:hmm:

[PBL]

Dynamic pressure!

Well, yes, for the loading, and therefore IAS. But what about flutter? My oracle for CS 25 is on vacation, and I can't put my fingers on a part which would require that high-lift devices be tested up to limit IAS within the entire envelope (but I haven't read every line of the nearly 500 pages).

PBL

[john_tullamarine]

Hopefully the Beakster will not be upset at my having undeleted his thread deletion.

Two reasons -

(a) generally we prefer not to delete threads - while the OP's need may have been met, others may find the details useful

(b) this is an interesting topic and we have a number of appropriate specialists in the Tech Log sandpit who may well jump in with pertinent comments. PBL's concern is one such ..

One of the problems which follows from a given operator's choosing to do something a little out of the ordinary is that the operator then assumes responsibility for any certification implications. It is for this reason that most will seek an NTO (no tech objection) from the OEM which addresses such certification matters.

The underlying problem is that certification implications may not be at all obvious to the layman trying to make sense of AFM wordiology.

[FE Hoppy]

AMC 25.345(c) High Lift Devices (Procedure Flight Condition)
1 En-route conditions are flight segments other than take-off, approach and landing. As applied to the use of high lift devices the following flight phases are to be included in en-route conditions:
– holding in designated areas outside the terminal area of the airport, and
– flight with flaps extended from top of descent.
The following flight phases are not to be included in en-route conditions:
– portion of the flight corresponding to standard arrival routes preceding the interception of the final approach path, and
– holding at relatively low altitude close to the airport.
2 To apply CS 25.341 (a) gust conditions to CS 25.345(c), the speeds VFC and VFD should be determined for the flap positions selected in en-route conditions.

[galaxy flyer]

I'm trying to figure a need to descend fully configured from FL280 :hmm: :confused:

Even the tac folks in C-17 weren't doing that when I left AMC. But, I fear for the structure doing so.

GF

[Metro man]

I understood that flap life was based on them being used for take off, approach and landing only. What happens next time you select full flap and they seperate from the wings because the amount of stress they have been subjected to is far in excess of their design limits ?

Where I fly we may be told to reduce to minimum clean speed or given holding whilst a long way out. Closer in, speeds which require flap extension may be given.

[PBL]

FE Hoppy,

thanks. I searched for "high-lift" rather than "high lift", forgetting engineers' English.
(c) If flaps or other high lift devices are to be
used in en-route conditions, and with flaps in the
appropriate position at speeds up to the flap design
speed chosen for these conditions, the aeroplane is
assumed to be subjected to symmetrical manoeuvres
and gusts within the range determined by –
(1) Manoeuvring to a positive limit load
factor as prescribed in CS 25.337 (b); and
(2) The discrete vertical gust criteria in
CS 25.341 (a). (See AMC 25.345 (c).)


I do note that the dynamic loading criteria with flaps down in en-route flight do not so obviously correspond directly with those to which the airplane has to be certified flaps-up. I also note that there appears to be no definition of V_FC and V_FD in CS 25.

PBL

[FE Hoppy]

PBL:
Yes I had a look for Vfc and Vfd just to confirm my understanding that it is V cruise with flap and V dive with flap.

I didn't find it!

[Tee Emm]

However, flap limits are based on IAS which as FEH says is to do with dynamic pressure and load on the surfaces.

Theoretical question. Apart from oxygen use, why is it when (ejecting) parachuting say from a high altitude of 35,000 ft, it is common to delay opening the parachute until below 10,000 ft. At time of ejection at 35,000 ft the IAS could be low and the dynamic pressure therefore also low. The initial rate of descent would be high of course and the TAS would be much higher.

[Mad (Flt) Scientist]

The parachute may not deploy cleanly in a low-Q environment, which might be an incentive to wait until there's enough airpseed to ensure it works properly.

To take an extreme example, if you tried to deploy a chute while standing still, I suspect there'd be a fair risk of something getting tangled.

[Mad (Flt) Scientist]

@FE_Hoppy and PBL

To add to possible confusion, there is of course ANOTHER "Vfc" which is the maximum speed for demonstration of flight characteristics, and lies between Vmo and Vd. It's used in, for example, 25.143(g).