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-80´s 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 can´t 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 (http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20081201X44308&ntsbno=DCA09IA014&akey=1)

[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 I´m 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. :confused: 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....:E

[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]

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]

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]

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.

[divinehover]

Am I correct in assuming that each engine manufacturer and engine generation use different methods to determine EPR. If so this would reinforce my belief that EPR isn't a valuable tool for pilots to use. Can I reliable interpret the EPR information from the RR Trent 556 on the A346 I fly one day in the same manner as the EPR info on the A332's RR Trent 772's I fly fly the next?

[rudderrudderrat]

If so this would reinforce my belief that EPR isn't a valuable tool for pilots to use.
I don't understand how you can say that and claim to sit in the front right hand corner.
It's another engine parameter - would you prefer just one gauge showing N1 and hide all the rest?
It's brilliant for confirming whether or not you actually do have take off thrust, and very simple for setting a good approximate approach power, even if you have an engine out as Meikleour explained in post #12.

[divinehover]

My point is that the actual value indicated is virtually meaningless. Every other engine gauge on display give me a value which i can compare against a min and max value. I don't have a hassle with the needle indication but rather the number displayed itself. That is why I suggest the EPR value should be replaced by a corrected % to be meaningful. So for example I would like to know that at FL410 the thrust set is 77% of what that engine can produce under those conditions. A number like 1.050 means zip. Surely the technology is around to display such info?

[rudderrudderrat]

Hi divinehover,

Your engines will be flat rated to ISA + ? (my old -22Bs were to ISA +15 degs C). So maximum thrust was always 1.526 EPR, & MCT was 1.419 up to 28 degs C at sea level (Simples?)
When it was less dense, the maximum EPRs reduced because we were now limited by another parameter usually Fan speed or TGT.

What could be simpler?

We used to Flex EPR down for reduced thrust take offs - but we knew our Max and MCT limits because we had to set them manually. You probably use FMS to control them now.

In the cruise we bugged up MCT from tables for the density altitude (in case it was needed), and expected cruise EPR using weight, density Altitude, cost index / Mach no. etc.

So for example I would like to know that at FL410 the thrust set is 77% of what that engine can produce under those conditions.
We had RPM limits which exceeded 100% as the engine was developed. So 77% of what? It's just a another number.
Is your TGT expressed as a % age of melting temperature?
Are your V speeds expressed as a ratio to V stall?

[Old Fella]

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

Not always so, the RB211's use EPR gained from comparing the ratio between P1 (Inlet Pressure) to P Int (Integrated pressure of core exhaust and fan outlet pressure). This gives a true indication of thrust being delivered.

[Denti]

It's another engine parameter - would you prefer just one gauge showing N1 and hide all the rest?

That would be (nearly) the boeing way on the 737. No EPR to start with, you get a big round dial with N1 and numeric EGT values. And thats it. Everything else is hidden and only pops up if something isn't right.

[aviatorhi]

Oldfella... seriously!?

Was it too hard to read the rest of the post? I only say that EXACT thing in the last paragraph.

[FullWings]

I think it's the variation in EPR ranges with environmental conditions on some engines that makes the number itself fairly useless.

RR Trent 895 max climb EPR ranges from 1.132 at a hot sea level to 1.518 at 43,000'. To fly level at 270kts, 15,000' and 160T GW you need 0.978... I can only quote these figures because I have the QRH displayed next to this posting. The numbers do not lend themselves to intuitive use, to put it mildly.

Comparing to something like the JT8 on the "classic" 737. 1.0EPR meant zero net thrust, 1.8-2.2 was full power. If you lost an engine on approach when you needed 1.3EPR on two, then 1.6 (twice the decimal part, plus one) would be a very good ballpark for the remaining motor. I can't remember now but there was another simple formula like taking 17 off the gross weight which would give you a very reasonable initial setting for a F30 3deg approach.

Point is, using N1 for a quick gross error check on takeoff, you're looking at c.85% on a full derate. Anything less than that should raise an eyebrow. Getting closer to MTOW, it's going to be into the 90's, probably 96-97% at the max.

Yes, of course you can get the books out and look up what kind of EPR to expect given various parameters but that sort of shows the lack of information from the original units used. Might as well be calibrated from blue1 to yellow15 with a limit of "sausage"... You get a power gauge on a Veyron (0-1001bhp!) and power reserve on a Rolls-Royce (100%-0), why can't we have a Thrust-o-Meter(TM) calibrated 0-100%?

[barit1]

In the very early days of jets, a direct force measurement (strain gage) was attempted but it was fairly useless on buried engines because it did not include inlet forces.

But podded engines are another matter - maybe Mr. A or Mr. B could give some insight whether measuring thrust mount or pylon strain is feasible.

Understand, though, that available physical thrust is a function of altitude - so tables or a corrected readout would still be necessary. :8

[Old Fella]

Aviatorhi. Apologies, did not mean to offend. I guess when the opening comment seemed incorrect I read no further.

[A33Zab]

In addition to FIS's reply:

A380 uses the THRUST parameter, here shown with eng#2 in degraded N1 mode.

Engine windmilling = 0% Thrust, TOGA = 100% Thrust (Bleeds off)

http://i474.photobucket.com/albums/rr101/Zab999/A380Display.jpg

[BraceBrace]

EPR is just a calculated controlling parameter
N1 is just a calculated controlling parameter
IEPR is just a calculated controlling parameter

The engine generates thrust and needs to be protected, it needs to be kept within limitations. As mentioned before, the limitations change and the highest possible generated thrust changes as well (temperature and pressure depending).

EPR, IEPR and N1 values always need to be corrected for current atmospheric conditions, so to a pilot none of them actually make sense. One has an andvantage (don't know if it's EPR or N1, should have to go back in the books though). We might think N1 is more adapted to our pilot environment, but it isn't really. Otherwise nobody would wonder why N1 and N2 values can be greater than 100%. 100% does not mean a damn thing. I can understand a pilot says "EPR doesn't mean a thing anymore". But so does N1. Say we need full takeoff power. Today that might be 95% N1, tomorrow it can be 105%.

EPR is still perfectly usuable for fan jet engines. On low bypass ratio engines the parameter might be more understandable for us, pilots, yes. But that has never been the goal :-).

BTW, even fuel flow can be used. Never flown the 737-200 with the JT8D in real, but in the simulator IAS/2 was your fuel flow. 250kts? 1250kg/h :-).

[barit1]

I surmise from the above discourse that the Trent series no longer use IEPR (per RB211) which always seemed to be very well liked by pilots. Does anyone have insight re this decision?

[ImbracableCrunk]

By rudderrudderrat:
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.
That, to me, seems like an argument for using N1.

[Beeline]

Im hoping that post 32 is a graphical representation only.

Thats a serious margin in that sort of temperature with packs and NAI on!

Lets hope it isn't heavy or high too! lol!

[rudderrudderrat]

Hi Beeline,
That's a serious margin in that sort of temperature with packs and NAI on! Lets hope it isn't heavy or high too!
I read it differently. It looks like it's taking off using reduced thrust and is below thrust reduction altitude. The assumed temperature is 59 degs C, whilst the ambient is probably below +10 degs C and in icing.

Hi ImbracableCrunk,
That, to me, seems like an argument for using N1.
If you really don't want to know your effective thrust - then stick with N1.

Cockpit Design: EPR v/s N1 indication « The Flying Engineer (http://theflyingengineer.com/flightdeck/cockpit-design-epr-vs-n1-indication/)
"The advantage with EPR is that it is a pressure ratio, and is indicative of engine thrust. Simple physics will tell you how: Force (Thrust) = Pressure multiplied by the Area of application. According to Boeing, EPR is more directly related to, and a much better indicator of thrust than the compressor speeds (N1 and N2), and therefore is more advantageous in terms of accuracy to utilize EPR to control engine operation."

[Beeline]

On second glance yes, Flex-temp adjustment.

Interesting concept of thrust percentage. I'm surprised they didn't integrate N1 and EPR into that parameter using both values. Or have they? Not yet been on the 380 type course.

[FullWings]

"The advantage with EPR is that it is a pressure ratio, and is indicative of engine thrust. Simple physics will tell you how: Force (Thrust) = Pressure multiplied by the Area of application. According to Boeing, EPR is more directly related to, and a much better indicator of thrust than the compressor speeds (N1 and N2), and therefore is more advantageous in terms of accuracy to utilize EPR to control engine operation."
Yes, it tells you what the thrust is more accurately but the numbers are not easily remembered and change wildly with external conditions.

You're taking off on a LR sector at near MTOW. When you line up and press TO/GA, the engines settle down at 1.213EPR. Is that good or bad? Ask the guys at Melbourne...

[rudderrudderrat]

Hi FullWings,

I've just read that incident report at Melbourne: http://www.atsb.gov.au/media/3532364/ao2009012.pdf and couldn't find anything about the lack of awareness of EPR mentioned.

What is mentioned is the take off weight error of about 100 tons wasn't trapped with:
Flight plan Expected TOW,
INIT B flight plan expected Zero Fuel weight entry,
green dot speed difference.
Larger than expected flex temp figure.

Even if the crew didn't have a "handle" on EPR, - N1 was still displayed.

[FullWings]

What I'm trying to say is that you get used to checking that achieved EPR = commanded EPR, not that the numbers make sense on their own. If the commanded EPR is *grossly* wrong (full derate where there should be hardly any, as above), it doesn't stand out in the same way it would with N1 as the controlling parameter. Yes, it's a good habit to look at the N1 as well as the EPR but when we're tired or in a hurry, "computer says OK" often ticks the mental boxes: all those needles lining up with the little bugs, quite compelling.

If you were in a Cessna and got 2200rpm instead of 2500rpm, you'd know there was a problem and reject the takeoff. If the gauge read 1500 one day and 3000 the next, for the same power, it wouldn't be so clear cut.

[barit1]

Re EK :rolleyes:@ Melbourne: When a 100 tonne mass error isn't evident until 2+km down the runway, there's little hope, is there? :rolleyes:

[Old Fella]

The item link below is another example of non-appreciation of EPR's and Bug settings. The SIA B747-400 scraped the tail for about 500 metres, staggered into the air and, thankfully, made a safe return after having the TOW entered 100 tonnes below actual. No one noticed the TOW and LDGWT were almost exactly the same when transcribing the data. The APU was almost hanging out the damage was so severe.


Crew mistake led to SIA tailscrape (http://www.flightglobal.com/news/articles/crew-mistake-led-to-sia-tailscrape-175617/)