Flap Limiting Speeds - IAS vs TAS

[Genghis the Engineer]

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!

The SR71 supersonic issue is a red herring, since I can't think of any current commercial aircraft which is going to have opportunity to lower flaps or gear above M=0.88.

At lower Mach numbers, where most of us live, this is all about EAS, which maps via CAS to IAS with the corrections being PECs and compressibility - the variability in both of which should be small (Depending upon PECs, correction from EAS to IAS in a part 25 aeroplane will be 10-20kn reduction at M=0.8 and FL280, decreasing with speed and altitude). It is EAS which determines structural loads, TAS is only an intermediate and I'd not bother thinking about it for other than navigational purposes.

G

[John Farley]

I'd not bother thinking about it for other than navigational purposes.


Fully agree G - unless you are one of those people that write sales guff.

While IAS and the dynamic pressure it represents is without doubt the prime stress producing number, in the very small print there may be trim change effects associated with mach number or possibly even flutter considerations which require an altitude limit. In the smallest print of all an altitude limit may represent the envelope of testing or "don't exceed than son as we simply have not tried it"

[Genghis the Engineer]

Moderators note: the original poster deleted this thread after JF's post with the message "question answered". Since there was nothing embarrassing, unpleasant, misleading or defamatory (so far!) I've decided to abuse my moderators powers and undelete it on the grounds that it was an interesting question and I can just sense various people sharpening their keyboards for a good old fashioned technical argument (or I may just have enjoyed John agreeing with me in a public place!)

Genghis
Moderator
(who also gets annoyed with sales guff listing speeds in TAS rather than the much more informative EAS/CAS/IAS)

[Kerling-Approsh KG]

Cat, these are pigeons...

Pigeons, this is a cat...

TAS, CAS, IAS, EAS, etc, are all parameters with which we feel familiar. Most of the time, they may usefully guide us to avoid damaging our aircraft and achieving our objectives of safe and efficient flight.

However, given the advances in technology of recent years, I'm struck that we still rely upon 1940s metrics when 2010 might give us dynamic indication which tells us more clearly what our aircraft is doing.

Following the accident to the THY 737 at AMS, I was struck by the fact that as I make the approach in my 737-800, or whatever, it tells me my speed, rate of descent, N1, configuration, etc... But not whether I will make the TDZ or not.

Can't we take a step forward?

[BOAC]

it tells me my speed, rate of descent, N1, configuration, etc... But not whether I will make the TDZ or not. I cannot see your point! Talking AMS, if you actually LOOK at your speed - which is below Vref and reducing - do you really need software to tell you the facts of life? What happened to FLYING?:confused:

[Kerling-Approsh KG]

OK, my post was a result of some very lateral thinking indeed... And I now call to mind gliding days, when I had instruments that dealt with energy rather than the individual elements of energy...

It's energy, range, and height that tell us what we're up to. Even the most modern aircraft don't have a 'You're not going to make the TDZ' alert, yet landing short is a significant problem. By the time you're low enough for G/S alerting and/or slow enough for stickshake, your energy is way out of kilter.

I wonder how the Shuttle does it..? Must ask...

[barit1]

The most impressive display of energy management I can recall is Bob Hoover in the Shrike Commander. Feather both, slow roll, loop, touch down and high speed turnoff, roll right up to park next to the P-51. All without further energy input. :eek:

Yes, I know the shuttle does it too, but with redundant computers managing the whole affair.

[ICT_SLB]

KG,
There are transport aircraft with such instrumentation - although whether it's being used in that mode is debatable. Any type with a HGS or HUD installed will have the Flight Path Vector (FPV) which will show very clearly if you haven't got the energy to make the TDZ - just look through the circle.

Generally transports will be in an Approach mode such that the pilot's task is to keep the FPV within the guidance - "making doughnuts" as our project pilot, Alain LaCharite, used to say - so it's not being used for that purpose but even a non-pilot (such as me) can see when the energy's not there. For example, Windshear alerting is normally only enabled below 1500 feet agl but in a sim one of my colleagues decided we needed to get the full benefit of a DFW shear at 2000 feet agl. It was easy to see the energy build up at the entry to the microburst even without the EGPWS such that the GA was relatively straight forward - although I'm still thankful the motion was switched off....

[hello1]

My concern here is not so much the IAS/TAS split - yes you are actually going faster at the higher TAS but the dynamic pressure and therefore the force on the flaps varies with the IAS. Not sure why some aircraft have an altitude limit but it may be a function of mach number (wild speculation).

My concern is about fatigue on the flaps and in particular the mounting structure. The flaps were probably not designed to be extended at close to their limiting speed from cruise altitude down to landing and this practice may well have a fatigue impact over time. Key point is to make sure that the engineers understand how the aircraft is being used and determine what might require additional or more frequent inspection.

[janrein]

@ Ghengis' quoted poster (still reading?) and all

AFAIK all limiting airspeeds are IAS (or EAS, for those flying faster) as it is all about aerodynamic and structural loads, except the VNE which is a TAS since aerodynamic damping relevant for flutter is a function of relative vertical and horizontal true airspeeds, which is why VNE in terms of IAS (EAS) is decreasing with increasing density altitude.

Now reading some of the posts, perhaps there is a coupling of flap configuration and flutter related parameters, which might explain a TAS aspect in the flap setting limits.

@ KAKG

Well yes, gliders use the TE instead of VS as airspeed is intentionally varied to maximise performance, unlike powered aeroplanes that are supposed to keep parameters largely constant, hence no point for TE.

Gliders BTW when instrumented for cross-country flying and glider races do have that feature that you are referring to, airfields within gliding range are marked green, airfields not (yet) within range are marked red and a lot more sophisticated data computing and presentation, all calculated accounting for wing profile including hand-adjustable profile deterioration due to wing contamination (insects on the leading edge, rain), winds as measured for altitudes passed while thermalling, etc. But then again, normal operating procedure for gliders is power off, and I imagine SOPs for powered aeroplanes should be centered on power-on. And was THY 737 at AMS not due to overreliance on automation already? OTOH, navigation and approach aids that do provide more insight while avoiding the trend to take the pilot out of the loop may be valuable additions.

The Shuttle does use glider-like glide-path calculation and planning, together with an MLS approach guidance system, allowing curved approaches allowing more flexible variation of flight path length.

Interested to hear when you receive more specific details about Shuttle approach management.

jr

[WeekendFlyer]

Hello1,

I think your concerns could be valid, but only if the stress engineers failed to allow sufficient margin for fatigue when calculating structural limit speeds! If they got the assumptions and maths right then you could fly around for years at VFE-5kt with no problems. However, if the fatigue case was based on a specific "time of use" for flaps fully extended, and your company operating procedures are significantly exceeding that assumption, then earlier onset of fatigue damage to a flap system component could be a possibility. However, one would hope that regular maintenance inspections would pick that up and deal with it. However, if a flight procedure is significantly different from "normal", then a chat with the aircraft design organisation or manufacturer by your company engineers would probably be useful. At least then an authoritative answer to the question could be obtained.

Regarding limiting Mach No, I would suspect almost all subsonic aircraft have a sufficiently low VFE that Mach considerations would not be a concern. If they were, the flight manual would specify VFE changes with height to ensure Mach related problems did not occur.

[Roger Greendeck]

Just wondering what the advantage is to configuring so early. Surely the extra drag of flying the whole approach configured would cost fuel and hence money, quite apart from any potential damage to the aircraft.

[sycamore]

It would be useful if we knew which aircraft type the OP was discussing,as I cannot think of a sensible reason to configure so early,or late,depending on your viewpoint,except for an emergency...,or one is doing specific type/procedure testing,in which case it will have been considered by the appropriate stressing/designer chappies...

[error_401]

Something about the Shuttle landing

NASA - Landing 101 (http://www1.nasa.gov/mission_pages/shuttle/launch/landing101.html)

http://www-pao.ksc.nasa.gov/kscpao/nasafact/pdf/LandingSS-2005.pdf

and simpler with some pics and grafics
HowStuffWorks "Space Shuttle: Return and Landing" (http://science.howstuffworks.com/space-shuttle7.htm)

[wiggy]

I cannot think of a sensible reason to configure so early

Khe Sanh or modern equivalent??

[UL730]

Perhaps this is the future?

'Flapless Flight' (http://www.cranfield.ac.uk/news/pressreleases/2010/page50603.html)

[hello1]

Weekend,

We have had some flap fatigue problems recently that suggest that exposure time is a factor and - as you suggest, it is probably because we are operating outside the envisaged operating model. The problems were picked up during maintenance inspections and so the system worked before anything bad happened.

H1

[stilton]

'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 must be missing something, why would you need to do that ?