Selected Temperature Derate (B744 FMC)
 

Hi, folks.
In various airline 747-400 ops manuals, it is written that the range of permissible entries on the THR LIMIT page for SELected temperature (for engine derate) is 0degC to 99degC. However, if, for example, I enter 99degrees, the value which appears on the SEL temp line is not 99, but something substantially lower (e.g. 59deg). There seems to be an imposed limit... but, from one aircraft to another and from day to day, this limit appears to change.

I'm wondering if the value which appears is dependent on TAT (or OAT) (e.g. TAT + fixed value(?) = displayed limit).

Any ideas?

Thanks.
Rgds.
Q.

The assumed temperature thrust reduction is limited to no more than 25% from the selected take-off rating level. This is to ensure that the FIXED derates of CLB1 and CLB2 (10% and 20%)will not be 'voided' by an assumed temp take-off derate being BELOW the armed values for derated Climb. In other words, it avoids the thrust being INCREASED when the CLB derate is activated at the thrust reduction point. This is done by automatically reducing the assumed temperature when entered, IF it would have produced that excessive derate.

CJ,

The 25% assumed thrust limit is a regulatory requirement. It is not set by the manufacturer, so I would therefore think that it has nothing to do with CLB power.

Mutt

The assumed temperature thrust reduction is limited to no more than 25% from the selected take-off rating level. This is to ensure that the FIXED derates of CLB1 and CLB2 (10% and 20%) will not be 'voided' by an assumed temp take-off derate being BELOW the armed values for derated Climb.

Hi, CJ.
By "no more than 25% from the selected take-off rating level", do you mean no more than 25% from the normal takeoff thrust(based on the actual OAT).... or no more than 25% from TO, TO1 or TO2?

I notice that when a large value, such as 99, is entered and the maximum allowable SEL temperature value appears(say 69degrees), the "armed" caret next to CLB automatically slides down to the CLB2 setting. This seems to suggest that the FMC is doing its best to avoid a thrust increase at the CLB point. I'm wondering what would happen if I then overrode the CLB2 value with CLB and CLB1... Would the value change?


"In other words, it avoids the thrust being INCREASED when the CLB derate is activated at the thrust reduction point. This is done by automatically reducing the assumed temperature when entered, IF it would have produced that excessive derate."

I don't see the harm in increasing the thrust for CLB (although I am aware that if the derated TO thrust is close to the armed CLB thrust, that the EEC's may not unlock from the TO mode).


[Mutt] Do you mean that the regulatory limit of 25% is for all 747 engines... or for all engines? A colleague thought he saw a greater derate selection on the 777.

Thanks.
Rgds.
Q.

Qavion

The 25% maximum thrust reduction is for all engines, and is defined in the FAR's.

It is permitted to reduce a fixed derate (TO1, TO2) by another 25%.

The thrust reduction selections on the 777 are identical to those on the B744.

You can input an assumed temperature of 99°C, but the FMS will reduce this to:

a: AFM limit, 54°C or 69°C (this is an option.)
b: 25% reduction off the rating in use.

I do not know if the FMS compares takeoff thrust with climb thrust for any other reason than to use the appropriate CLB thrust for the Takeoff rating in use.

Mutt

Thanks, Mutt....
More pieces of the jigsaw puzzle in place!

Cheers.
Q.

Qavion
Mutt has covered the 25% relationship, which, as he says is the maximum allowed. It can be modified to a lower figure (greater power) by the operator. As for the CLB derating, it changes to reflect the Assumed Temp applied and can be changed - without affecting the de-rated T/O power. My previous employer used to do just that, always using at least CLB1 even if the automatic de-rate went to CLB2.