A320 OEI ceilings.

[vilas]

Hi everybody
In A320 IN FLIGHT PERFORMANCE OEI Ceilings graph which is pretty linear sharply flattens between FL 250 FL 260. Between these FLs the ceiling varies 166ft/T while outside this range it is about 440ft/T. Why is that? Engine efficiency? I am unable to find any answer. Can some one explains this please?

[Cough]

Look at the max thrust available on the EWD next time you pass FL250... Ramps back quite a lot at this point and I guess the point is there aren't that many hills above 250, so its a cost benefit/certification issue.

[FlightDetent]

Vilas sir, I have no idea. Instead of engine core efficiency my initial thoughts would be towards overall aerodynamic performance, namely:
a) air mass density / compressibility - IAS-TAS-Mach relation at that region (some sort of an optimal point)
b) Rudder effectiveness / spoiler use related to a)
c) speed polar of the wing, as for different weights the OEI speed will be slightly different so perhaps reaching some sort of optimal alpha point with relation to max Cl/Cx or min Cx.

[vilas]

My observation was based on CFM engine. Which has the relight envelope also starting at 25000ft. and below. Surprisingly IAE2500 engine graph has a gentle curve in the 15000 to 20000 region but nothing significant.

[latetonite]

While you are discussing the EO ceiling from the FMS, please consider the real EO max altitude in the event of use of engine, or even wing anti-ice, or if you would fly high speed for some reason.

[vilas]

The OEI ceilings I have given are from QRH. Although for IAE engines there is no flattening of the curve between 25000ft. and 26000ft, the LRC ceilings are much lower in IAE. In IAE at 60Tons weight LRC ceiling is 5000ft. lower than CFM. I had earlier started a thread IAE VS CFM. So this is another handicap in IAE.

[Goldenrivett]

Hi vilas,

The same engine and wing is on A319, but there is no flattening around FL 250. The OEI ceiling and weights are identical for both variants around 60 tons and lighter, but the performance of the A320 is better at higher weights.

I think the reason is because the maximum thrust available of the engine is increased on the A320 below FL 250, so that at maximum take off weights, they have similar performance.

Above FL 250, they have similar maximum thrust available.
OEI A319.jpg (http://www.blackholes.org.uk/PP/OEI%20A319.jpg)

[vilas]

Hi Golden
I think what you say looks possible because it is only on CFM and on Airbus CFM is widely used on all variants