Why is EPR set by 60/80 KTS?

[moon11]

And,also
What happens to EPR on the take off roll?

Many thanks..

[Draculas Teabag]

On jet types with Autothrottle, for example the B744, at 65 knots the autothrottle mode changes from THR REF (thrust reference limit - determined by ambient temp, pressure and any derate/assumed temperature thrust reduction) to HOLD (A/T disconnected) to prevent any unwanted thrust lever movement caused by an autothrottle malfunction. So if thrust isn't set by 65 knots the throttles will stay wherever they are. THR REF is re-engaged when VNAV engages which is usually 400 feet above runway elevation.

Also you don't really want to be diddling with thrust levers when your belting down the runway at a great rate of knots, so you're better off setting thrust early and concentrating on keeping the beast on the runway!

EPR will usually increase on the takeoff roll if left alone as the power a turbine engine produces is a product of the temperature, pressure and velocity of the air entering it. I've never really noticed an increase in a jet(RR), but it is certainly noticeable in a turboprop engine such as the PT6.

[Chosun]

Hmmmmm,
EPR decreases with increasing airspeed, doesn't it? On Classic with P&W -7, if EPR was set to 1.44 TO thrust at 80 kias, then at VR this would have reduced to 1.42 (= G/A Thrust). If thrust was advanced to 1.44 at VR, an 02 overboost was being applied. Actual thrust increases with Ram effect, however................

[TR4A]

Takeoff EPR is based on OAT, Outside Air Temp, and the EPR is required to be set by 60 to 80 KIAS. As the the airspeed increases the the TAT, Total Air Temp, increases. GA power is takeoff power that is adjusted for RAM rise. Normaly slightly lower than Takeoff power.

[mutt]

The amount of thrust produced by an engine decreases with speed.

Takeoff performance is based on a specific amount of thrust, if this thrust isnt set by 80 kts, then it isnt achievable and you are not guaranteed your takeoff performance.

For example.

Mach / .00 / .10 /.20 / .30 / .40 / .50
Deg F / Thrust (lbs)

29.0 / 93169 / 83629 / 75589 / 69114 / 63248 / 54866
59.0 / 92861 / 83339 / 75305 / 68860 / 63006 / 54607

Draculas Teabag,

Your automation is a result of the above, the A/T system has nothing to do with the requirement to set takeoff thrust by 80 kts, it just helps you to do it.

Mutt.

[v953]

We Awll know that Mitt, but what in Heavens Name do all those Numbers mean to a Pilot?

U a bookworm/wannabee/reggy spotter?

Go out a bit more often

[quid]

T/O EPR settings are a static value. That's why they must be set before Ram effect occurs. If you keep a close eye on the EPRs on your takeoffs, you will notice that around the time you're raising the gear, they will be approx. .04 lower that they were at the start of the roll. (This will vary, of course, with a/c type and ambient conditions). If you push them up, you will be overboosting.

GA EPR is designed to take Ram effect into account. You will notice (if you compare TO and GA EPR settings) that GA will be lower, even though both will produce the same thrust.

Mutt gave you some good examples of differing thrust with differing conditions.

[mutt]

Quid,

Not sure about your 8’s, but on Boeings takeoff thrust isn’t a static value, it’ based on a speed of approximately .10 Mach, hence the requirement to set it before 80 kts.

V953,

Welcome to Pprune, now if you want to be more helpful try answering the question rather than attacking people!

Mutt.

[BEagle]

In the dear old Vickers Funbus, one commands one's man to set take-off thrust. One's non-handling pilot merely confirms that the NH is greater than the minimum value; if it isn't, one orders maximum thrust - and one then devotes one's full attention to flying the dear old thing.

To fiddle with the noise levers during take-off - how very lower deck! One has a servant for such things!

[flyin74]

Thanks for your replys guys!