View Full Version : EPR reading

20th May 2006, 02:20
Could anyone explain to me what happens to EPR indication as an aircraft accelarates on the take off roll past 80 knots and why?

Thank you

20th May 2006, 02:22
Pt2 increases, and thats the denominator.

21st May 2006, 15:20
Thank you for the information.

23rd May 2006, 00:08
How is engine pressure ratio determined? Seems very abstract to me.

23rd May 2006, 01:39
There is a probe at the intake to measure intake pressure and one in the exhaust to do the same. EPR is the ratio of the latter to the former.

23rd May 2006, 01:55
Keep in mind both pressures are measured via aneroid transducers, so they are absolute pressures. Thus when shutdown at SL, both are measuring 14.7 psia, and the ratio 14.7/14.7 = 1.00.

At takeoff, EPR is 1.65 to 1.8 or so depending on the engine. If the ratio reaches 1.89, the nozzle is sonic velocity (very very noisy!)

23rd May 2006, 06:43

Is there also something to do with inlet size versus outlet size? and also the amount of bypass? Older engines (JT3 etc) had higher EPR values than modern big fans. If I remember correctly the JT3 on 707's used to be about 2.02 on take off (ISA etc) versus engine I use today which is 1.54 on take off...

Still learning....

23rd May 2006, 13:22
Correct me if I'm wrong -

I think all the P&W machines measure the EPR numerator in the core nozzle. With the straight jets (JT3C, JT4, also GE's CJ805-3 & CJ610) this makes great sense because that's where ALL the thrust is generated. I'm a bit surprised at the number being as high as 2.02, but so be it.

However - with a fan engine, esp. a high-bypass fan, the EPR at the core nozzle raises a degree of uncertainty. The core nozzle has had much/most of its energy removed by the LP turbine in order to drive the fan. Thus the core EPR is down to the 1.54 range. The EPR if measured on the fan nozzle is likely a bit higher than that; I note that the Roller integrates the two values (IEPR) which again makes good sense.

GE introduced the concept of rating the engine by fan rpm (N1) on the CF6 about 1970. It has served them well, & has carried over to the CFM56, CF34, & GE90 donks as well. Pilots of mixed fleets (starting with KLM 747's - P&W plus GE versions - around 1977) seemed to have no transition problems.

23rd May 2006, 14:55
Unfortunately, some modern engines seem to have a very strange EPR derivation. On the RR Trent as fitted to the 777, you can be at a constant speed and altitude with <1 on the EPR gauges...:confused: On other types I have flown that would mean drag. Personally I use N1 as it seems to be far more related to the thrust produced.

In the good ol' days on the clockwork 737 EPR really seemed to mean something - you could get very close to the required thrust setting by doing some mild mental arithmetic with the a/c mass. If you lost an engine, you just took the bit after the decimal point, then added it to the remaining engine. 2 x 1.4 = 1 x 1.8. Simple! Doesn't seem to work like that now...:sad: