ISA and max altitude and coffin corner

[md-100]

with a SAT above ISA how the max altitude and the buffet margin behave?
what about below ISA??

[JABBARA]

Hi,
Max altitude drops down with ISA+, or vice versa (self evident).

Coffin Corner limits does not change. In another word Lower and Higher Buffet onset speeds remains same. (ref. Airbus FCOM 3, General Limitations, Buffet Onset Chart. The version I have A 330, 3.01.20 P4a)

Regards.

[BOAC]

Buffet onset speeds - for the benefit of those without such Airbus charts, can you elaborate on what you mean by 'speeds'- eg IAS, CAS, TAS, Mach?

[JABBARA]

BOAC,

I tried to put the chart here, but not able. If later I can, I will.

Input of Chart: Load Factor (margin), Weight, CG and Pressure Altitude. I believe they are all self evident for you.

The output comes as Mach No only, as the lowest and highest limit. And this makes sense to me.
Because:
The lowest limit practically leaves a margin to stall (by default 1.3 g limit). And we know at near Sea Level altitudes where compressibility effect (mach effect) does not kick in yet, the stall AOA is fixed and that is why stall speed as KCAS (dynamic pressure) is nearly independent from altitude. However at high altitude where compresibility takes effect, the stall AOA is variable (reduced as compared to SL) so stall KCAS greatly influenced for same conditions (e.g for same weight). Therefore expressing the Lowest margin as Mach is more accurate and more sensible for pilots because that is what we monitor at high altitudes.

The highest limit till cross over altitude (for 330 tahat is 30000 feet PA, where 0.86 mach equals to 330 KCAS and not effected by ISA DEV) is the dynamic pressure limit, that is why it is given as KCAS (or precisely KEAS). But what I know at high altitude the buffet reason is not the dymanic pressure (namely KCAS or KEAS) but the shock waves on the wing. Therefore expressing the highest margin as Mach is also more sensible for pilots.

Regards.

[BOAC]

-s'ok - I was just confused by the word 'speed' but if you mean Mach that is ok.

[Mephistopheles]

I posted this in R & N but it was deleted???

Anyway, some years ago whilst operating an A340-300 in the Far East we flew into some light cloud at night with no turbulence. All of a sudden the Capt's IAS dropped to around 120kts whilst the F/O's & stand by continued to show reasonable indications. At the same time the TAT & SAT indications increased to something around -20 c at cruise FL(can't remember exactly but must be FL330+). Since an we were already aware of the problem(we had been issued an incident report rgarding a Cathay A330) we weren't in too much shock. Airbus' write up about the problem was that the heating in the TAT probes,at times, may be insufficient & lead to the probe icing up followed by the ice rapidly heating up giving false TAT & SAT indications.

[safetypee]

Re: “…the heating in the TAT probes, at times, may be insufficient & lead to the probe icing up followed by the ice rapidly heating up giving false TAT & SAT indications”.

See the thread http://www.pprune.org/tech-log/375784-pitot-icing.html#post4963257 and technical links, some of which describe the conditions associated with TAT probe icing. Note the number of incidents at the edge or beyond the icing certification envelope and their association with adverse weather as describe in the incident.
TAT probe icing most probably accounts for reports in several posts (R&N) describing a ‘rapid’ rise in TAT, or TAT near zero. The limits of atmospheric physics makes the ‘hot air’ bubble theory most unlikely.

A point of interest is to consider if the reported airspeed speed changes originated from coincident pitot tube icing, or if the ADC uses TAT in a Mach computation, which is corrupted, causing a change in the indicated Mach and hence changing the EFIS airspeed.