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