perfos A320

[avionneta]

does anybody know why in the airport analysis charts, the MTOW still increases below T ref. if it increases, it means more thrust is available but below Tref thrust is supposed to be constant (flat rated). Is that flat rate really flat or the real curb is a slightly sloped line (quasi flat ?)

[Old Smokey]

The thrust may well be constant, as are the V1/Vr/V2 speeds which are CAS for a given weight. Accelerate-Stop and OEI Climb Gradient performance depend upon TAS, e.g. for a RTO, stopping capability depends upon the TAS, which after considering wind component, enables us to know the speed to be dissipated in said RTO.

If the Flat rated temperature, was, for example 30 degrees at Sea Level, and V1 was 140 KIAS, TAS is 143.6 Kt. At 10 degrees temperature the TAS for the same IAS at V1 is 138.8 Kt. As the energy to be dissipated in a RTO depends upon speed SQUARED, the energy at the 30 degree takeoff is 1.07 times (107%) of that at 10 degrees. Thus, as temperature falls from 30 to 10 degrees, RTOW as limited by RTO will be 7% higher.

Similar discussion arises with respect to OEI Climb Gradient, where, for a given Rate of Climb, Gradient will be higher for a lower TAS, thus allowing greater Takeoff Weight for engines producing identical thrust.

NOTE - When using Flex or Assumed Temperature Thrust, the difference in TAS for the same speed is usually NOT considered, leaving some small conservatism. For the Limiting Weight case, it is DEFINATELY considered.

Regards,

Old Smokey

[FlightDetent]

In addition to what OS describes, shoud the TKOF profile be limited by obstacles, same thrust at same IAS in colder air will also provide more lift (1/2*density*...) which is reflected in RTOW charts as well.

FD (the un-real)

[avionneta]

thanks guys !
however you're in the wrong check-list and the question is what I wrote initially in my post

[FlightDetent]

does anybody know why in the airport analysis charts, the MTOW still increases below T ref. if it increases, it means more thrust is available but below Tref thrust is supposed to be constant (flat rated).

Ok, got it now. Yes, PLTOW (performance limited TOW) increases with temperature drop. It is not because of a thrust increase (there is no such thing - owing to the flat rating, as you correctly say) but rather to other variables in the equation. Thrust same? Yes - no changes. Lift same - No, a larger force available. Quicker acceleration to V1? Yes - lower energy at V1 - so less braking distance needed.

At the end: at rated temperature, say, 35 deg C ....... max permissible TOW 100 (%). ... at 05 deg C ..... same thrust, but better aircraft performance! Once all accounted for - higher max permissible TKOF weight!

FD (the un-real)