airport analysis and full thrust takeoffs
 

Have heard some conflicting opinions with regards to using the airport analysis for a takeoff when not using the assumed temperature method of engine derate. ( Ie full thrust )

In some cases the limit weight will be significantly greater than the actual weight on the day.
In this case do you use the speeds associated with the full rated takeoff or do you use the speeds associated with the actual weight.

I have flown for operators who have used both methods but never read anything in the manuals that covers this scenario.

Any info, references or explainations would be helpfull.

Thanks

OVER THE TOP,

A caveat in advance, Company Policy may vary, and Company Policy is the operating method approved by the relevant authority in your AOC, presumably CASA in your case.

From the performance perspective -

(1) The RTOWs have verified that your Actual Gross Weight is less than the Limit, so operation is legal at both your present weight, and at the limiting weight shown in the RTOWs......SO......

(2) In normal operations, use the speeds for your Actual Gross Weight. These should be obtainable from the AFM / QRH / FMC etc.

(3) [This may not be Company Policy - If not, it should be] If your Takeoff is in conditions where windshear protection is desired, use the highest Takeoff speeds permissible, i.e. the speeds provided in your RTOWs. An extra caution here, sometimes the RTOW exceeds Structural Limit as these are Performance Limits, if this is the case, use the speeds for Max. Structural Weight. (For the caution given here, operation should still be OK for speeds applicable to a weight at the performance limit greater than structural limit, but CASA might take you to task for using speeds for a weight outside the approved envelope of operating weights)

Best Regards,

Smokey

Thanks Smokey.
Current Company policy does specify to use the speeds for the weight, but was interested to know what others do.

The official aircraft documents I have conveniently leave this scenario unexplained.

Speed for the actual weight is surely the commonsense answer.
Unless severe windshear is present (which changes the goal posts a bit), there is no operational advantage to boring down the runway to Vr speeds in excess of those for the actual weight.
Where Improved Climb speeds are required for performance considerations then obviously the discussion ends there.

I believe that accident studies reveal that engine failures are rarely the cause of aborts near V1 but that tyre failures are the real worry. I recall a figure published by Boeing that 27% of over-runs that turned into accidents were caused by tyre bursts or tread problems. Presumably the higher the ground speed, the greater is the risk of tyre wear and tear. In any case it would be preferable to have the safety blanket of a lower V1 which is not the case where the aircraft is deliberately and unnecessarily left on the ground for no pressing operational reason.

Speed for Actual weight.
I had a problem with a previous employer who insisted on using speeds for actual temp.
Often at low TOW this would result in the A/C getting airborn before placarded V1 if the handling pilot didn't hold a lot of nose down.
Not much fun if it takes off then the PF notices and pushes the nose down and the jet thinks you have just landed again?!*"%

What got me thinking about this scenario in the first place was Vmcg and Vmca considerations.
Would a takeoff using the lower speeds for the weight cover the Vmc requirements for a takeoff at the higer power setting of rated thrust ?

Have however since been advised that airport analysis takes into account the possibility that you may elect to go rated thrust at some stage during the takeoff.

In any case a glance at the V1 /mcg tables in the QRH makes it unlikely that it would ever be a limitation.

Thanks for your help.

(a) derate is the "same" as having a lower thrust engine available for the takeoff

(b) flex is reducing thrust for the takeoff on a given thrust engine - Vmc/Vmcg are based on the rated thrust - not the reduced thrust set by flexing.

(c) Vmc/Vmcg, if they are to be a problem, will be so for low weight takeoffs at min speed schedules