GE engines not displaying EPR

[dixi188]

It's Engine Pressure Ratio, (intake to exhaust) so with no thrust the ratio is 1 to 1 . If half thrust is 1.3 to 1 then full thrust will be1.6 to 1 approx.

[FullWings]

Currently flying with both, I have to say I prefer N1. If you’re going off on a 12hr flight, you know 80% N1 is not a good thrust reference (or at least you should double-check) but 1.276 EPR could be OK or not as it is affected a lot by ambient conditions. With engines synched on N1, you don’t get the phasing vibration you get when it’s trying to match EPR giving a slight mismatch in RPM. Even worse with 3 or 4 power units all slightly different.

Lots of different sensors contribute to EPR and there are lots of ways of it being wrong; it is difficult to conceive of a scenario where N1 is not correct but >0, so it’s more obvious that it’s not right. If you have serious blade damage, all bets are off at that point and you get what you get. On my current steed, 0.97 EPR cruises you around at 250kts in some scenarios so it seems to have lost the direct connection to thrust and is just another number. Did I say I prefer N1?

[Vessbot]

Originally Posted by FullWings

With engines synched on N1, you don’t get the phasing vibration you get when it’s trying to match EPR giving a slight mismatch in RPM. Even worse with 3 or 4 power units all slightly different.

Whoa whoa wait, in your EPR planes is all that's available to synch off of, or did someone actually decide that that's a good idea?

[Check Airman]

Originally Posted by wiggy

I suspect tdracer means you to ignore the leading 1, since it's effectively zero thrust.(?)..

So do that and the thinking is that .3 is half .6

Having probably made an idiot of myself I'll get back in my box now.

We made the same assumption. But what’s max EPR on that day? If it’s 1.6, I can understand that, but I’m my experience the EPR limit is usually something like the square root of pi.

I don’t think that assumption is correct though. Because then, they could just configure the display to ignore the leading “1”, then shift the decimal point 2 places to the right. Oh wait....never mind

[tdracer]

Originally Posted by Vessbot

Whoa whoa wait, in your EPR planes is all that's available to synch off of, or did someone actually decide that that's a good idea?

On the 767 and 747-400 - with FADEC EPR engines - the autothrottle will align EPR across the wing (obviously with N1 engines it will align N1 across the wing). It's not possible to automatically align N1 with EPR engines with the autothrottle active (even if you try to manually stagger the throttles to align N1, the 'trimmer' will override the throttle stagger to align EPR).
On the 777/787, it is possible to align N1 with EPR engines and the autothrottle active, although it may require manually switching the FADECs to Alternate N1 mode (not sure about that last part - it's been a long time since I worked the 777). That's not an option on the 767/747-400 - the autothrottle won't engage with EPR engines in Alternate N1 mode.
That being said, with FADEC EPR engines it's normally not much of an issue - there tends to be minimal N1 variation with EPR with FADEC. That wasn't the case with the old hydromechanical control JT9D-7. On the JT9D powered 767, the N1 'beats' could be super annoying for those sitting near the wing. The JT9D controls needed to manually adjusted ('trimmed') on a regular basis (every month or two) to adjust the stator vane, bleed, and power lever/N2 relationships. The N1 beat issue was bad enough on the JT9D/767 that we developed what became known as 'super trim' - where the stator vane schedule was adjusted (within the allowable tolerance) to align N1 with EPR.

[Vessbot]

This has been an interesting thread, the gist I'm getting from it is that EPR isn't all that it's cracked up to be. I've never flown with it, but until now I was attracted to the idea of it being a more direct and 'pure' thrust indication, loosely speaking kinda like AOA is to airspeed.

[FlightDetent]

Originally Posted by Vessbot

This has been an interesting thread, the gist I'm getting from it is that EPR isn't all that it's cracked up to be. I've never flown with it, but until now I was attracted to the idea of it being a more direct and 'pure' thrust indication, loosely speaking kinda like AOA is to airspeed.

And then there is the HMI design of the gauge itself. On the A. models the readouts somehow deny much of the reasonable use.

[Fursty Ferret]

Yeah, I just used N1 for everything other than setting take-off thrust when I was flying the A320. To add to the thrust measurement topic, I think you can also assume that an EPR of less than 1 is an indication that the engine is generating more drag than thrust (would be good to get confirmation of this point).

[FullWings]

To add to the thrust measurement topic, I think you can also assume that an EPR of less than 1 is an indication that the engine is generating more drag than thrust (would be good to get confirmation of this point).

The RR Trent 895 certainly doesn’t subscribe to that. From the 777ER QRH: 15,000’, 270kts, less than 200T gives required EPR < 1, so whatever the number means, it is not directly representing net thrust.

[Check Airman]

Originally Posted by Fursty Ferret

Yeah, I just used N1 for everything other than setting take-off thrust when I was flying the A320. To add to the thrust measurement topic, I think you can also assume that an EPR of less than 1 is an indication that the engine is generating more drag than thrust (would be good to get confirmation of this point).

Concur. Other than the initial thrust setting, I’m only vaguely aware of the EPR.

[tdracer]

Originally Posted by Fursty Ferret

To add to the thrust measurement topic, I think you can also assume that an EPR of less than 1 is an indication that the engine is generating more drag than thrust (would be good to get confirmation of this point).

I can say that an EPR less than 1.0 almost never means 'negative net thrust'. With an EPR less than 1.0, yes the pressure of the core exhaust is less than the inlet, but it is much, much hotter - and hot air expands, so it can still be producing thrust. Further, since the vast majority of the thrust comes from the fan, and it's almost unheard of for the Fan Pressure Ratio ("FPR" - pronounced 'Feeper') to be less than 1.0.
It is extremely rare that an engine that is still running (idle or above) is not providing some positive net thrust.

[hans brinker]

Originally Posted by tdracer

I can say that an EPR less than 1.0 almost never means 'negative net thrust'. With an EPR less than 1.0, yes the pressure of the core exhaust is less than the inlet, but it is much, much hotter - and hot air expands, so it can still be producing thrust. Further, since the vast majority of the thrust comes from the fan, and it's almost unheard of for the Fan Pressure Ratio ("FPR" - pronounced 'Feeper') to be less than 1.0.
It is extremely rare that an engine that is still running (idle or above) is not providing some positive net thrust.

Today I learned. Thanks

[4djd9]

Regarding the RR Spey, in the mid 70s I flew F-28s for our company, and I seem to remember the takeoff card had TIS as primary takeoff power setting. I´ve forgotten what the gauge for power indicated, but I´m pretty sure it was thrust related and not RPM related. Anyone out there care to refresh my memory ?

[Pugilistic Animus]

Expected Fuel Flow would also work to serve as a good indicator of thrust, good crosscheck
AirFlorida may have been saved if the pilots did that.

[Centaurus]

Expected Fuel Flow would also work to serve as a good indicator of thrust, good crosscheck
AirFlorida may have been saved if the pilots did that.

Refer to Post 4. While I agree with the point of fuel flow check, from personal experience I can say that when such an event tales place such as the Air Florid Potomac crash, the PF tends to only glance at the engine gauges of EPR early during the takeoff run. There is no way in my experience the PF could closely check the fuel flow and make a meaningful assessment that the needles would have shown it to be even slightly below the expected figure even if the correct fuel flow for the power was present.

There was snow on the runway during the Potomac accident so keeping straight on the centreline on a slippery runway would have been an absolute priority by the captain even if he could distinguish the centreline under the snow. Assuming for the sake of argument the captain is PF, his total concentration is on the runway centreline and correcting quickly for the slightest deviation - especially if any significant crosswind exists.

Any glance at the engine instruments by a PF is a quick glance of one second maximum then back to the centreline. Providing that glance sees EPR and N1 for both engines are all parallel and there is no sudden pull to one side signifying a significant thrust imbalance, then he will assume operations are normal. That is true for a night takeoff as well where the cockpit lighting is low. The PM is watching two things. Firstly that the aircraft is going straight down the runway and secondly a glance at the engine parameters. if the needles of each engine are all parallel as expected he too assumes there are no discrepancies. It is only when you see a significant split between both engines that alerts either pilot to a problem.

I had two incidents where in each case on the takeoff roll the copilot noticed a sudden large split between the EPR and called out. The first occasion was at 30 knots. I had felt no swing and the aircraft was accelerating normally. I hadn't even looked at the EPR at that low speed because I was concentrating on the the centreline. As soon as the copilot called my attention to the split EPR I looked and saw one at 2.10 EPR and the other at 2.24 EPR. The takeoff was rejected at low speed and the aircraft taxied back for investigation by maintenance. OAT was 30 degrees C it wasn't icing..

The second event was more disturbing. Around 100 knots the copilot called something about "overboosting on No. 2" and tried to pull back the No 2 thrust lever from under my hand. There had been no swing. I glanced at the N1 and both gauges showed equal readings of 100%. In other words there was nothing wrong with the engines. By holding firmly to both thrust levers I was able to stop the copilot from continually trying to drag back the No 2 thrust lever as he was convinced the engine was overboosting.

I said "Set N1 at 100%" and still he tried to over-ride me on the thrust levers despite both N1 showing correct at the planned 100% N1 for the P&W JT8D-17 engines. During the climb the problem fixed itself and the No. 2 EPR returned to the expected figure.

EPR as a primary thrust parameter is fine in theory but when it has been known to give an occasional erroneous reading, one starts to be wary of trusting it implicitly.

[tdracer]

Originally Posted by hans brinker

Today I learned. Thanks

You are very welcome. I rather enjoy threads like this, where I can share some of that 40 years of experience.
Helps compensate for some of the BS that goes on in other threads...

[Pugilistic Animus]

Centaurus, I do understand what you're saying. In fact, it is reinforced by fact that FF isn't on the primary EICAS. Instead, it is​​​​ kind of of out of sight. But I'm obsessive and I liked to take a glance at Fuel Flow just to ensure TLAR Because of the concept of Thrust Specific Fuel Flow. I understand rapid acceleration and the necessity of keeping the airplane on course and I also understand that N1 is right there where I needed it as a back up to EPR still I always glanced at Fuel Flow...just for my own edification.

[Three Wire]

<EPR as a primary thrust parameter is fine in theory but when it has been known to give an occasional erroneous reading, one starts to be wary of trusting it implicitly. >

A long time ago now, I flew a pointy nosed fast thing powered by one J52. The Primary thrust reference was EPR calculated before start from the QRH T/O Reference card. The other gauges were N1 and fuel flow. The Flight Manual recommended use of EPR checked at brakes release, but also said that if the calculated N1 was OK, the flight could go.
Having only flown a Viper powered machine prior to this type conversion I was quite happy with that. Came the day, and at Brakes Release, the EPR didn't quite make the bug, but N1 was good and the fuel flow also looked good. So I took off and drilled holes in the sky for an hour. back on the ground, I wrote up the EPR and explained my reasoning to the Shift PO. He hit the the roof, but the Chief Mech walked in, asked me a few questions, turned to the PO and said "Sir was right, the book says....". I learnt from that and EPR throughout my career was just a high tech guide.

[Fursty Ferret]

I can say that an EPR less than 1.0 almost never means 'negative net thrust'. With an EPR less than 1.0, yes the pressure of the core exhaust is less than the inlet, but it is much, much hotter - and hot air expands, so it can still be producing thrust. Further, since the vast majority of the thrust comes from the fan, and it's almost unheard of for the Fan Pressure Ratio ("FPR" - pronounced 'Feeper') to be less than 1.0.
It is extremely rare that an engine that is still running (idle or above) is not providing some positive net thrust.

Thanks. Always good to learn something new.

[wondering]

As long as exit velocity is faster than inlet velocity, the engine will produce thrust. As for the general thrust equation: https://www.grc.nasa.gov/WWW/K-12/BGP/thrsteq.html I reckon, if manufactures had reliable means to assess mass flow and inlet/exit velocities we would be using that for setting thrust.