Alternative to EPR

[divinehover]

Flying along the other night in a A346 (although this is irrelevant) I glanced across to the EPR gauges which read something like 1.050. I realized that the actual value didn't mean much to me. The relative position of the needle on the gauge means something but not the actual number. What am I missing here? This made me wonder if there might not be a better way of indicating actual thrust vs idle and max available thrust. Say for example, a thrust percentage gauge which compensates for alt/temp ect. I've flown EPR gauges on many different types (B727, B744, A319. A340) so I'm even surprised by my own question. Perhaps I'm an N1 man by heart.

Any thoughts?

[Field In Sight]

The A380 has an indication called THRUST which I always thought should be the way Airbus should represent engine output.
It would help the cockpit to be more common across types.

FIS

[dixi188]

IIRC early BAC 1-11s had P7 gauges but later ones had EPR gauges calibrated in %.
Used to be called lie meters as you set a number on the gauge before T.O. so they showed 100% when thrust set.
Can't remember what they showed in the cruise though.

GE CF6 just uses %N1 for power setting.

[OPEN DES]

Hi DH,

EPR is a good measurement of thrust as you know. The problem lies in the higher by-pass ratios nowadays which means that the actual EPR as a number has become pretty meaningless operationally. As you said: 1.050 is pretty meaningless. I remember that on the MD-80īs for example EPR was around 1.80-2.00 or so for take-off. Pilots used to have rules of thumb for EPR numbers for different phases of flight, and used EPR operationally as their main thrust parameter.
In the newer high-bypass ratio engines the EPR has become a pretty meaningless control parameter for the pilot. Airbus still refers to N1 rules of thumb even for EPR equipped aircraft. Problem is that the governing parameter is EPR which is directly proportional (give or take) to thrust lever angle. N1 however is not directly proportional to EPR/thrust lever angle, which makes manual thrust control slightly more difficult. i.e. TLA not linear with N1.
My thoughts anyway.....

I canīt think of any alternative to EPR... Not familiar with the A380, what does the Thrust parameter indicate?

Regards

O.D.

[Capn Bloggs]

EPR can be misleading, as the Crew of Air Florida 90 found out.

The only real measure of thrust is the speed of the big fan on the front of the donk!

[lomapaseo]

Flying along the other night in a A346 (although this is irrelevant) I glanced across to the EPR gauges which read something like 1.050. I realized that the actual value didn't mean much to me. The relative position of the needle on the gauge means something but not the actual number. What am I missing here? This made me wonder if there might not be a better way of indicating actual thrust vs idle and max available thrust. Say for example, a thrust percentage gauge which compensates for alt/temp ect

Why do you care?

Isn't airspeed more critical in cruise?

[rudderrudderrat]

the EPR gauges which read something like 1.050. I realized that the actual value didn't mean much to me.

You are correct!
That's idle power.
EPR = 1 = zero effective thrust.

@Capn Bloggs,

EPR can be misleading, as the Crew of Air Florida 90 found out.

Especially since they had failed to turn on the engine anti-icing. The FO tried to tell the Capt. that something was not right with the engine power settings (N1 was way too low). EPR is only one parameter to be checked - it's not the sole indicator.

The only real measure of thrust is the speed of the big fan on the front of the donk!

Er no - that will only tell you how fast the big fan is rotating.
The actual thrust will also depend on forward airspeed and air density.

[divinehover]

If EPR 1 equals zero thrust (which is not idle for a jet) how come have I seen EPR of 0.9-1.0 in the cruise? RR Trent 772 I think.

[rudderrudderrat]

Hi divinehover,

I don't know how your EPR is displayed, but this Boeing B777-232ER (B777) used conventional figures of about 1.25 EPR in the cruise.
"the right engine EPR dropped from approximately 1.25 to 1.1."

Boeing B777-232ER (B777), N862DA, operated by Delta Air Lines as flight 18

[OPEN DES]

If EPR 1 equals zero thrust (which is not idle for a jet) how come have I seen EPR of 0.9-1.0 in the cruise? RR Trent 772 I think.

I am by no means an expert on EPR. Have only flown an EPR equipped Jet for 600 hours or so before moving on to N1.

Normally EPR 1.00 is what you would have at idle on the ground. I always understood that EPR below 1.00 in flight is at idle with the pressure rise caused by the Ram Air effect. Correct me if Iīm wrong please....

[FullWings]

If EPR 1 equals zero thrust (which is not idle for a jet) how come have I seen EPR of 0.9-1.0 in the cruise? RR Trent 772 I think.

In some installations, EPR actually has a deducible meaning and can be used with rules-of-thumb. On things like the Trent, it appears to be just another random number with EPR<1 giving net thrust. Much easier to look at N1 which is what most of the tables in the QRH use, funny old thing!

EPR also changes markedly with altitude & temperature, which makes it difficult to perform quick sanity checks on things like T/O thrust unless you're the Lucasian Professor of Mathematics...

[Meikleour]

Divinehover: EPR had one quite useful property as I recall from the B747-200 with RB211 engines - since the EPR increment over 1 related to the thrust produced then, say, if approach power required was say 1.25 EPR then this gives a thrust equivalent to + 1.0 EPR ( 0.25 x four engines) therefore on three engines the EPR required would be 1.33EPR ( +1 delivered by three engines) Similarily for a two engine approach the EPR would have to be 1.50EPR on the two running engines. This relationship does not hold for N1s. Other aircraft use total fuel flow as a similar approximation since thrust relates to fuel flow.

An EPR reading of less than 1.0 for instance in the descent is merely confirming that the combination of fan + core is actually producing drag!

Hope this is helpful to you?

[Cough]

As a driver, I love the security being driven by an engine that takes EPR as the controlling parameter. I watch N1's to understand what is going on, but I'm assured that at takeoff thrust that is exactly what is being produced.

In an N1 aircraft, at takeoff N1, then we can be assured that the fan is spinning at the correct speed. However, that fan may be nearing the end of its life, as such the thrust produced may be slightly degraded.

All subject to having the anti ice on as required....

[aviatorhi]

Cough,

That's all good, but EPR measures the difference in pressures between the intake end and the core exhaust, not the fan exhaust.

I believe a big "contributor" to the decline of the EPR gauge was that more of the "thrusting air" is now coming from the fan instead of the core (ie. fan speed is a more relevant indication than core pressure differential). For instance, a takeoff power setting on a JT8D-17 is around 2.17, while on a -217 is 1.91. The cores are essentially the same on both.

What would be useful is what is only found on the RB211, an IEPR gauge which takes a combined reading of core and fan air pressures.

[Cough]

Dohh! Nice point, well made!

[Capn Bloggs]

Originally Posted by Rudderrat

Especially since they had failed to turn on the engine anti-icing. The FO tried to tell the Capt. that something was not right with the engine power settings (N1 was way too low). EPR is only one parameter to be checked - it's not the sole indicator.

Unfortunately, EPR is the main parameter used (by both ATS and crew). That's why they speared in! The FO thought something was wrong but couldn't convince the captain; had he been able to say "we need 92% but only have 88%", the accident would never had happened. The EPR was correct; the N1 and therefore thrust wasn't, but couldn't be readily accepted by the captain because he probably didn't have any "normal" N1 numbers in his head.

Do you know what you N1 should be on any given takeoff, or do you trust the EPR?

The only real measure of thrust is the speed of the big fan on the front of the donk!

Er no - that will only tell you how fast the big fan is rotating.
The actual thrust will also depend on forward airspeed and air density.

Of course it does, as does EPR. You missed my point. N1 is a physical number not reliant on any blocked pressure probes, anti ice or whatever; if the N1 is spinning at the required speed, then you will have the thrust required, much more so than EPR, even considering "forward speed and air density".

In my books, N1 is a far more reliable thrust indicator than EPR. Now what is it supposed to be?!

[aviatorhi]

Do you know what you N1 should be on any given takeoff, or do you trust the EPR?

Not sure about others, but my T/O data cards are written as follows (for this very reason).

2.15 2.17 2.15 || 99%
2.00 2.02 2.00 || 94%

The PNF will trim the throttles out to the desired EPR then the Engineer will fine tune the the throttles on the roll taking into account all indications. Though, most of the times we leave ourselves enough takeoff margin where setting all 3 slightly low will still leave us room to spare.

[Dan Winterland]

Different engine manufacturers take their EPR guage references from different sections of the engine. If you fly a P and W powered 744, the EPR settings you see for similar stages of a flight are very diffrent from a RR powered 744. And if you fly a GE powered 744, you just have N1 displayed, which personally, I prefer.

Having spent the last 7 years flying an an Airbus type, the only time I look at the EPR guage is on the take off roll when PNF when checking the power is set. And all I do is make sure the guages read what is displayed as the reference thrust on the upper engine display. Our take off performance calcualtions don't even give us the reference EPR, so we trust the dispaly is accurate. I have no idea about the EPR settings for other stages of flight - if I'm using manual thrust I reference the power I need based on N1, the speed strip and speed trend vector. I used to fly an older bypass turbojet engined aircrat where the Flight Engineer had a P7 guage measured in inches of mercury. The only time they referenced this is on the takeoff roll to ensure the engines were performing as required. If the P7 was down a bit, he gave it a bit more lever movement. (The FEs had their own thrust levers in the VC10).

So, I would agree. EPR is largely irrelevant except to make sure the engines are performing as you need on takeoff.

[Turbine D]

EPR verses N1

On modern turbofan engines, thrust is more accurately controlled by setting fan speeds, particularly engines with large fans (GE90, etc.). This is because all of the airflow is pumped by the fan rotor. The core rotor (compressor) only handles a portion of the airflow, a lesser amount of the total, as the fan size increases in diameter. That said, as the fan speed and core speeds are coupled, thrust can be accurately modulated by scheduling core speed as a function of PLA (Power Lever Angle). A speed curve can be generated. This is a curve of corrected fan speed verses corected core speed, or corrected core speed verses thrust.

Earlier turbofan engines used core speed control exclusively. In other words, the pilot set EPR including fan speed with the throttle. Speed is affected by various parameters like engine component efficiencies and stackups (relationship of one component to another), altitude, mach number, schedule tolerances and deteriorations. Therefore, core speed is not an optimum manipulated variable to set fan speed. Override controls have been established to allow for direct scheduling of fan speed in a limited PLA range, usually in the takeoff or max climb PLA regions. Fan speed is typically controlled as a function of T2 (engine inlet temperature) which results in corrected fan speed.

Obviously, this may be debatable as historic practices differ from one engine manufacturer to another and even differences in basic engine design, two spool verses three spool.

[rudderrudderrat]

EPR shortfall

Hi Capn Bloggs,

if the N1 is spinning at the required speed, then you will have the thrust required,

With RB211 - we had something called EPR shortfall. When trying to set the required EPR, sometimes we couldn't quite make it due reaching N1 or TGT limit first due engine degradation. Hence there was a performance penalty to apply.

Without EPR, you could be happily setting the required N1 without realising you weren't actually achieving the required thrust.