Optimum Flex temp for takeoffs using TOPCAT and stopping margins

[787flyer 633]

Hi,

 

I have been wondering if there is such a thing as an optimum Flex Temp that can be selected when presented with a range of viable flex temp options all other things being equal, in TOPCAT?

Is there a minimum stop margin for specific aircraft that comes into play or is this completely pilot discretion or what other logic comes into play to determine which flex temp is best suited for departure?

With a little trial by error I have been selecting take-off parameters that give at least 600m margin in good weather conditions.

 

There's not so much info about this specific topic re stopping margins and Flex Temp selection I could find, but did find this little piece of info from an Airbus training which stated:-

 

"According to FCOM, use the configuration that gives the highest flex. If both of them provide the same level of flex thrust or if you cannot flex, use the one with the highest flaps setting. This may change in specific conditions. "

 

I would be grateful if someone could provide some insight as to what needs to be considered and how to select the best flex temp when there is more than one Flex Temp available?

I am aware that various field/aircraft limitations and weather criteria play a big part in this but I'm not completely clear.

 

Last, do those same criteria apply to de-rated thrust departures?

 

Thanks,

 

[skelsey 765]

Hi Mark,

 

The general rule is to select the greatest possible derate (i.e. the highest possible Flex temperature) as this will offer the greatest saving to the airline in terms of engine maintenance. Remember the stopping margin displayed in TOPCAT already has a number of safety margins built in, there's no need to add your own margins on top of the margins already included in the calculation.

 

There may be airline-specific SOPs that govern takeoff flap settings and derate usage, generally to mitigate against inadvertently selecting the wrong configuration for a given departure.

 

Do keep in mind the big difference between de-rated and assumed temperature (flex) thrust reduction -- namely that when you de-rate, your minimum manoeuvre speeds are calculated based on the de-rated thrust value, whereas when you apply only an assumed temperature reduction the manoeuvre speeds are calculated based on full takeoff thrust. The practical application of this means that if you have an engine failure, if you are using a de-rate the maximum thrust you can apply is the de-rated takeoff thrust -- applying full TOGA may cause loss of control. Using an assumed temperature (only), you may apply full thrust because that's what Vmca/Vmcg are calculated from.

 

Many airlines have procedures in place prohibiting use of assumed/de-rated thrust on contaminated runways -- bear in mind, though, that this generally means significant quantities of standing water or snow/ice on the runway, rather than just a few drops of rain (most modern grooved & porous runways can be treated as dry unless/until braking action is reported as poor).

 

Hope that helps!

[787flyer 633]

Hi Simon,

 

Many thanks for the helpful answer. This is what I was looking for plus more.

 

Cheers,

[skelsey 765]

No worries Mark.

 

I should also add that in terms of flap setting, contrary to the Airbus quote above, most airlines (in my experience) tend to use the lowest possible flap setting for departure -- as this will provide better climb performance (less drag). More flap will provide a shorter take-off run at the expense of climb performance. To offer some examples, as far as I'm aware BA SOP is to use 1+F on the Airbus unless limited by runway length -- likewise the B767 fleet now uses 5 everywhere (whereas 15 used to be standard, with 5 only used for hot & high airfields). Other airlines may vary!

 

Simon

[787flyer 633]

OK got it! Seems to vary between aircraft and airlines. I can see where the logic comes in. Thx for explaining. :wink:

 

Cheers,

[JAT 185]

So on a B767 for example,which would it be better to use  de-rated  or assumed temperature.

 

Thanks

[skelsey 765]

 

 

So on a B767 for example,which would it be better to use  de-rated  or assumed temperature.

 

It depends -- on the requirements at hand, the engines and the options the airline has chosen -- not all have or use fixed de-rates.

 

If you have both, you can if you wish use a combination of the two -- de-rate and assumed temperature -- but you need to be aware of the limitations.

 

De-rate can be (in very basic terms) thought of as bolting a less powerful engine on. This has the advantage of reducing the thrust (typically by a fixed percentage, usually 10% or 20%) and therefore saving the engines.

 

However, the main advantage of de-rating is to reduce Vmcg on contaminated runways. V1 is limited by Vmcg: basically, if an engine fails at V1 you need to be travelling fast enough to generate enough airflow over the control surfaces to keep the aircraft straight using the rudder whilst you accelerate to Vr.

 

If the runway is slippery, Vmcg (using full rated thrust) may be quite high -- because when one engine fails, you would have all the thrust of the other engine trying to turn the aircraft, which must then be counteracted by the rudder.

 

If you bolt smaller engines on (de-rating), you can use this lower thrust setting in order to calculate Vmcg, which will normally result in a lower figure. This means that you may be able to take off, or lift more weight, out of that slippery runway by using a de-rate.

 

HOWEVER, what you must be aware of is that if you lose an engine at V1 whilst using a de-rate, you cannot push the other thrust lever to the stop to get more thrust on the remaining engine. You may lose directional control, as Vmcg has been calculated based on the lower, de-rated thrust.

 

Assumed temperature can (in very basic terms) be thought of as simply not pushing the thrust lever all the way up: you're keeping some thrust 'in reserve' as it were. This is significant, because your V speeds (particularly Vmcg) are then calculated based on the full rated thrust. This means if you have an engine failure at V1, you can quite safely push the thrust lever up.

 

The law says you can only reduce takeoff thrust by a maximum of 25%. This places a limit on the maximum assumed temperature that can be entered.

 

If you have a really long runway, you can achieve more than a 25% reduction (legally) by using a combination of de-rate and assumed temperature. As mentioned above, this is because a de-rate selection becomes the new thrust limit. Therefore, you can (say) select a 20% de-rate, and then apply an assumed temperature to reduce the thrust by anything up to a further 25%. However, the same rule applies if you lose an engine at V1: the maximum thrust you can select is the de-rated thrust limit (i.e. you may remove the assumed temperature, but the de-rate).

 

Airlines have different procedures around this. The Big Airline I'm familiar with doesn't have fixed de-rates at all on the B767 fleet, so it's assumed temperature only. On the B747, standard procedure is to apply an assumed temperature only (i.e. no derate) unless very light (i.e. an empty ferry). The exception is on a contaminated runway, where a de-rate only (with no additional assumed temperature reduction) may be considered in order to improve payload capability (for the reasons outlined above).

 

Hope that helps.