Why EPR?

[barit1]

oceancrosser:

I don´t think Boeing "removed EPR" from the 737 Classic. Rather they chose an engine, CFM56 that used N1 as primary power parameter in line with GE philosophy. The CFM56 predates the 737 Classic by almost 10 years.

And the CF6 used N1 when it first flew on a B-52 testbed - late 60s. http://www.aviastar.org/pictures/usa/boeing_b-52.jpg

[Uplinker]

I think if EPR gauge scales were shown as a percentage, instead of 1.387 or whatever, us pilots would find them much easier to use.

We want ballparks which we then fine tune. Setting 55% N1 is easy. Setting (and remembering) 1.432 or whatever is not.

Lose the decimal point and the digits after it. Have the scale read -10% to 110% say.

[Check Airman]

I think if EPR gauge scales were shown as a percentage, instead of 1.387 or whatever, us pilots would find them much easier to use.

We want ballparks which we then fine tune. Setting 55% N1 is easy. Setting (and remembering) 1.432 or whatever is not.

Lose the decimal point and the digits after it. Have the scale read -10% to 110% say.

I believe that's what Airbus did with the "Thrust" indication.

I don't see the point of the EPR indication either.

[Uplinker]

Ah, right. It is not on the 320/321/330 I fly, but it sounds sensible.

[tdracer]

We (Boeing) have done a lot of work over the years with regard to a generic "thrust" indication - something that would go from 0 to 100%. Short story is when it works, it's great, but the failure modes are nasty.
BTW, the reaction to Air Florida was not to get rid of EPR, it was to make the inlet probe heat automatic (e.g. JT9D on the 767, PW4000 on the 767, 747-400, and 777), RB211-524G/H (767 and 747-400) and Trent (777 and 787). Loss of inlet probe heat will automatically result in alternate (N1) mode operation.
Oh, and EPR less than 1.0 is far from meaningless (and doesn't mean negative thrust). At cruise airspeed, EPR can easily get as low as 0.7 with an engine making positive thrust due to the ram component (I don't think I've ever seen a scenario with a running engine that the fan pressure rise or ratio (FPR) wasn't positive - and that means positive thrust). Further, even if the primary exhaust pressure is the same as ambient (static) pressure, the much higher temperature of the exhaust means positive thrust.

[FullWings]

For me, the issue is non-intuitive EPR values. For example, on the GE90-115, full thrust is 100-105% over a large range of altitudes and temperatures. If you’re on a derate, <85% probably means something is wrong. In the circuit, F5, 55% and 5degs NU is a good starting point.

With EPR on the RR Trent, who knows? 1.372, 1.594, 1.193...? It’s not very memorable and changes much more with environmental conditions. Is 1.228 a good thrust setting for a derate? Gross error checking isn’t as easy.

At the end of the day, the AT doesn’t care if the units are nano furlongs per ounce second squared or walnuts per light year: if it’s slow it adds a bit and if it’s fast, it takes some away.

[FGD135]

EPR represents the thrust you actually have.

N1 represents what you should have.

[Cough]

Originally Posted by tdracer

We (Boeing) have done a lot of work over the years with regard to a generic "thrust" indication - something that would go from 0 to 100%. Short story is when it works, it's great, but the failure modes are nasty.

I presume the issues were overcome to produce TPR on the 787?

[A Squared]

Originally Posted by Romasik

EPR values less than 1 are not meaningless. They mean that the thing is producing drag, not thrust. Something that you never get from N1 indication. Could be usefull in understanding what's the difference between idle thrust and dead engine. Of course you need to know EPR at the same speed on dead engine. We may check it in the SIM.

Well, yeah, You'd think. But, If I'm reading this correctly, there's at least 2 posters speaking of having all EPRs <1 in steady state cruise. Which indicates thrust being produced at an EPR less than one. Dunno, I don't fly jets, so I'm just trying to follow the conversation.

[FullWings]

Here we are from the QRH unreliable airspeed tables for RR Trent:

15,000’, 270kts, 160T, EPR = 0.978, N1 = 64.2%

I can’t remember what the EPR is on a failed engine, I’ll have a look next time I’m in the sim.

[Meikleour]

Regarding EPR versus N1 for power achieved: I once had an experience of fan blade damage to two engine on a CFM56 powered A340. The resulting N1 settings required to make the approach were very much higher than usual since the two affected engines were not producing the normal thrust expected from the N1 readings.
When talking about the Trent engine EPR values, iirc these are Integrated EPR since the engine has three spools. So ,one frequently saw values less than 1 although I am quite sure the engine was not "sucking" at the time!!!!

[FlyingStone]

It's interesting that Airbus doesn't even mention EPR setting for unreliable airspeed or severe turbulence (IAE powered A320s) and uses N1 instead, even though this is said to be primary thrust setting indicator.

My personal opinion - EPR information is very nice to have, but N1 as primary thrust indicator/setting wins hands down. Very few failure modes and very reliable. In the very rare event that you have some engine damage (bird strike etc.), you could always refer to EPR that could give you some more clue.