Originally posted by PJ2 #604 (26th March):
*My thanks to HazelNuts39 for an extensive and informative discussion which helped me greatly in understanding this certification issue. Perhaps he may be coaxed into posting a suitable, brief discussion on VS1g and VLS certification (which is slightly different than VS certification)? I think it is germane.
I’m not entirely sure what PJ2 has in mind but, for whom it may interest, here are some further thoughts on what we’ve been discussing. I PM’d PJ2 because I doubted the speeds from the CBT in PJ2’s post #502: VLS=214kt and ValphaMax=196kt; i.e. VLS = 1.09 ValphaMax. On the other hand, PJ2 finds it difficult to believe that the airplane stalls at an angle as low as 6.5 degrees at FL 350, and I tend to agree with him on that. (Edit: The question that PJ2 and I still have is - what is the stall AoA in cruise?)
If we assume that the CBT’s VLS is defined by the stall, but alphaProt and alphaMax are somehow ‘adjusted’ for buffet boundary, we could estimate alphaMax(stall) as either:
VLS/1.18 = 181kt; alpha = 7.7 degrees, or –
VLS/1.23 = 174kt; alpha = 8.5 degrees.
For reference purposes, some elements that contributed to our discussion are repeated below:
Originally posted by PJ2 in #518 quoting from the FCOM:
V2=1.2 * 0.94 VS1g = 1.13 VS1g
VREF=1.3 * 0.94 VS1g = 1.23 VS1g
(...)
The AOM uses VS for VS1g.
PJ2 quoting from FCOM in his PM of 17th march
(HN39: only relevant part copied here):
VLS : Lowest Selectable speed.
Represented by the top of an amber strip along the airspeed scale on the PFD. It is equal to:
- 1.13 VS at take-off
- 1.18 VS when the flaps are retracted.
- 1.23 VS when in clean configuration. (It remains at this value until landing).
VLS is corrected for Mach effect to maintain a 0.3g buffet margin.
(
note by HN39: 0.3 g buffet margin means that VLS >= 1.14 Vbuffetonset)
Regulatory minimum speeds for performance calculation:
In the takeoff configuration: V2 >= 1.13 VS1g
In the enroute configuration (clean) VFTO >= 1.18 VS1g
In the landing configuration VREF >= 1.23 VS1g
Discussion:
‘VS’ in the FCOM is VS1g in regulatory terms, and is also known as ValphaMax. Thus the FCOM's definition of VLS is conform the regulatory minima for takeoff and landing, but differs for the clean configuration. This does not mean there is a conflict, 1.23 is a larger margin than the minimum required for enroute. VLS = 1.09 ValphaMax is not conform the FCOM nor the regulation. As suggested by CONF iture in #514, the CBT software may be inaccurate. Another possibility is that these speeds are governed by buffet margin rather than stall:
CONF iture #582 quoting from FCOM 3: The aircraft can also enter alpha protection at a high level where it protects the aircraft from the buffet boundary. (...) This value of alpha is not however the same as the values used at low speed (sic): alpha for alpha protection is reduced as a function of Mach so that a typical cruise value is in the order of 5°.