EPR BY 80KTS

[US Marine]

The cathay interview guide is great for questions, not so hot for answers!

Anybody know the real reason for setting EPR by 60/80kts on the take-off roll and what happens to the EPR on the t/o roll itself. I've tried to reason through this one and find some info but failed dismally.

Thanks in hope.

[WOK]

The target EPR has to be set with ref to some speed or other, at a given thrust setting it will be varied by speed. 80kts is the speed chosen to be fast enough that there's time to stabilise parameters and not so fast that a shortfall will cause a high-speed RTO.

As airspeed increases with throttles set the ram effect will vary EPR. (Increases, presumably). Modern, clever types (eg767) have a/t submodes which engage at 80kts (throttle hold)which prevent low level inversions etc from causing throttle movements for a period of time after t/o.

Most commercial a/c I've flown have had EPR gauges and I HATE THEM!!!!!!!!!!

[quid]

Actually the EPR reading will decrease when the airspeed increases during the t/o roll due to the ram effect.

If you compare the in-flight max power setting with the t/o max power setting, you will see about a .04 difference.

If the EPR is set later than +/- 80 kts., it is no longer a valid reading.

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[banjoyboy10]

What's the exact definition of EPR?

Is it Ratio of outlet (jet pipe) pressure to inlet pressure? If so how's it measured and where exactly on the face of the fan and in the aft section of the engine?

Thanks in advance :)

BB10

[CCA]

Rolls use Integrated Engine Pressure Ratio (IEPR) which uses Intake pressure (P2.0/T2.0) vs Fan exhaust (P1.3) AND Hot stream exhaust (P5.0) or P INT.

The P2 probe is at the top of the inlet.
The P1.3 probes are on the A frames, Upper left, lower right and the Interservices fairing.
The P5s are on the turbine bearing support struts.

I'll get back to you with photos.



[This message has been edited by CCA (edited 18 September 2000).]

[banjoyboy10]

Thanks CCA :)

[Centaurus]

It is well documentated that in the JT8D engine (early 737's) the EPR will over-read if the EPR inlet tube ices up. Thus on take off with engine anti-ice off, but with ice blocking the inlet tube (that is part of the EPR system) the cockpit EPR gauge will show a higher figure than normal. This may cause the pilot to pull back the throttles until the EPR needles read correct take off EPR. The danger of that is that less engine thrust will be produced. Worst case is that you may have less than even climb power. Potentially deadly on a limiting length runway. That is why monitoring of the N1-which indicates the real thrust, is so important.

What is not often realised is that it is not only ice that can block the EPR inlet tubes. Some years ago a 737-200 came close to going off the end of a short runway on an atoll in the Central Pacific. OAT 30 degC. Investigation revealed that both EPR inlet tubes were blocked with a combination of coral dust and insects. These could not be seen during a walk-around inspection. Fortunately the captain firewalled both engines when it was obvious that it was going to be a close shave. A simultaneous double EPR mis-read was unheard of - especially as the only Boeing reference to false EPR readings was in connection with icing conditions.

[gas path]

Try...... http://www.pprune.org/ubb/NonCGI/Forum3/HTML/000555.html

[Ex Bus Driver]

I don't know whether or not it has been previously mentioned, but I recall learning
a long time ago that when you set take-off power on a jet engine, all of the engine
gauges will be pointing to either the nine
o'clock or the three o'clock position. I
know that the JT-8D is 9 o'clock, and the CF-6 is 3 o'clock. In the case of the Air
Florida 737 that crashed out of DCA some years back, though EPR was set at 9'clock, all the other gauges were pointing to about 7 o'clock! As the probe starts to ice closed,
it will start to indicate a power setting higher than actual. EBD

[CCA]

P2.0 T2.0
http://members.theglobe.com/smcgoo/P2T2S.JPG
http://members.theglobe.com/smcgoo/P2CLOSEUP.JPG

[CCA]

P1.3,
http://members.theglobe.com/smcgoo/AFRAME.JPG
R/H 'A' frame.
http://members.theglobe.com/smcgoo/AFRAMECLOSEUP.JPG


[This message has been edited by CCA (edited 18 September 2000).]

[CCA]

P5.0

You can see the ports on the LE of the bearing support strut located at about the 7 o'clock position.
http://members.theglobe.com/smcgoo/P50A.JPG

[Centaurus]

EBD. Re the relative positions of the engine gauges in the 737 Potomac crash (blocked Pt2 tubes). It is not quite so simple as that. On the B737-200, the difference between planned N1 (say 95%) and the actual N1 (say 88%), is very little when you see the needles - especially if the cockpit lights are real dim as at night. Same with the fuel flow needles. The N2 needles also show very little visual distance between them.

We set up the anti-ice page on the simulator and select either Pt2 inlet blocked with ice - or sometimes we set up both Pt2 tubes blocked with ice. Very few pilots pick (on the take off run)the lower N1, lower fuel flow or lower N2 - although of course the planned take off EPR is set and clearly visible. All because there is very little angular change of needle reading.

Believe me, one or two fully blocked Pt2 inlets can easily be missed during the engine instrument scan during the take off run. Especially on a dark night with instrument lighting dim. I know - I have seen it happen in the real aircraft, as well as in a simulator. The fix is to set the initial power by using the N1. If the EPR setting is way out, then trust the N1 - not the EPR.

[natops]

epr on to roll must be set by 80kts because to check to/thrust and associated temp. All tables and charts are calculatedwith this value at this speed.

If you try to maintain calculated TO thrust setting after 80KTS, while the EPR is decreasing, thee is a good change that you will overtemp the engine. Because you keep increasing the thrust.

Good luck!
D.

[Ozgrade3]

So what reference is a go around EPR setting