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stilton
19th Jul 2020, 20:35
I’ve operated P&W, RR and GE power plants, all except GE display EPR and it is the primary thrust reference


Always wondered why GE chooses to not display this parameter, using N1 as primary instead ?

dixi188
19th Jul 2020, 20:50
If all the fan blades are there and undamaged, N1 will give a good indication of thrust. The early A300s had EPR gauges but they were removed at some point. They were on the FE panel so were not used for setting thrust. I guess having EPR was useful for cross checking other engine parameters. They did away with engine vibration gauges as well.
In the days of pure jet and low bypass engines you needed EPR of P7 or similar.
As to why GE went down this route I haven't a clue.

tdracer
19th Jul 2020, 23:09
N1 is a simple parameter that's far less prone to measurement errors than EPR. For EPR, you need highly reliable inlet and exit pressure measurement - including probe heat for the inlet probe (and the electrical power required for probe heat is significant - around 500-600 watts per engine). Further, heated pressure probes have still been known to ice up and give erroneous indications. With N1, you need to measure the rotor speed anyway so no additional instrumentation is required, and it's a simple, highly reliable measurement.
Newer EPR engines not only cross compare inlet pressure with aircraft total pressure, they calculate a 'synthetic' exit pressure based on other engine parameters, and if anything is out of a fairly narrow tolerance, the FADEC invalidates EPR and you set thrust based on - that's right - N1.
Now, there are some advantages to EPR - assuming it's accurately measured it correlates a little better with thrust then N1 does, and it's not as affected by high humidity as N1 (N1 power setting charts basically have to assume the humidity is very high - which can make a meaningful difference in thrust at high ambient temperatures and humidity. As a result, N1 rated engines need more margin and tend to 'give away' a little thrust relative to EPR engines (i.e. an average N1 will give you a little more thrust than an EPR engine at a specific thrust set). EPR for a specific thrust setting is usually independent of temperature - so for example takeoff EPR is a constant below the corner point temperature - where as N1 varies with temperature due to that messy "root Theta" term to convert Corrected N1 to Physical N1. With fan damage (e.g. birdstrike), N1 goes up while thrust goes down - EPR engines are much less affected by that issue.

It's worth noting, aircraft have crashed due to EPR measurement errors. I can't think of any that have crashed due to N1 measurement errors.

Centaurus
20th Jul 2020, 02:21
It's worth noting, aircraft have crashed due to EPR measurement errors. I can't think of any that have crashed due to N1 measurement errors

The Air Florida Boeing 737-200 takeoff crash into the Potomac River at Washington was a tragic example of EPR errors that were a contributory cause. See: https://en.wikipedia.org/wiki/Air_Florida_Flight_90

Plus, this writer had personal experience as jump seat observer during a night take off from a Pacific island in a 737-200. Both Pt 2 sensors were blocked by foreign objects ( insects and coral dust). Indicated EPR on take-off was 2.18 (JT8D-17 P&W engines). Actual EPR was probably around 2.10.

The difference in N1 between 100% (planned) and actual N1 95%, is so small on the gauge - around 5mm - that the crew never picked it with low instrument lighting on the dark night. Both engines EPR displayed 2.18 EPR, in other words significant over-reading from the true EPR. At the last few seconds of the takeoff run on the short 5600 ft runway the captain realised the aircraft would not get airborne by the end of the runway. An abort would have been fatal into the rocks and sea at the end of the runway.

The captain simultaneously firewalled the thrust levers and pulled back on the control column and was able to get airborne just below V1. Confirming - just below V1. The increase in acceleration at the moment of firewalling the thrust levers was very noticeable. A classic example of a correct instant decision to GO and firewall the thrust levers that resulted in the aircraft just scraping airborne. This incident showed the vital importance of cross-checking EPR readings with planned N1 during takeoff. If any doubt use N1 as the final arbiter. .

Three Wire
20th Jul 2020, 04:58
EPR was invented by and is proprietary to RR. When PW made a licence agreement with RR to produce engines in USA, the EPR technology was included.
hence why GE does not use EPR.
When the FADEC on a RR decides there is a problem, N1 becomes the default control instrument.
The airplane drivers do not need EPR to get airborne, but you do need N1. See Centaurus's illuminating post. After Palm 90, at least one operator decreed that N1would be calculated and bugged on one engine, The other was left at EPR.

Check Airman
20th Jul 2020, 05:31
N1 is a simple parameter that's far less prone to measurement errors than EPR. For EPR, you need highly reliable inlet and exit pressure measurement - including probe heat for the inlet probe (and the electrical power required for probe heat is significant - around 500-600 watts per engine). Further, heated pressure probes have still been known to ice up and give erroneous indications. With N1, you need to measure the rotor speed anyway so no additional instrumentation is required, and it's a simple, highly reliable measurement.
Newer EPR engines not only cross compare inlet pressure with aircraft total pressure, they calculate a 'synthetic' exit pressure based on other engine parameters, and if anything is out of a fairly narrow tolerance, the FADEC invalidates EPR and you set thrust based on - that's right - N1.
Now, there are some advantages to EPR - assuming it's accurately measured it correlates a little better with thrust then N1 does, and it's not as affected by high humidity as N1 (N1 power setting charts basically have to assume the humidity is very high - which can make a meaningful difference in thrust at high ambient temperatures and humidity. As a result, N1 rated engines need more margin and tend to 'give away' a little thrust relative to EPR engines (i.e. an average N1 will give you a little more thrust than an EPR engine at a specific thrust set). EPR for a specific thrust setting is usually independent of temperature - so for example takeoff EPR is a constant below the corner point temperature - where as N1 varies with temperature due to that messy "root Theta" term to convert Corrected N1 to Physical N1. With fan damage (e.g. birdstrike), N1 goes up while thrust goes down - EPR engines are much less affected by that issue.

It's worth noting, aircraft have crashed due to EPR measurement errors. I can't think of any that have crashed due to N1 measurement errors.

Thanks as usual for your insight. What are your thoughts on the technical advantages and disadvantages of the newer TPR measurement?

BDAttitude
20th Jul 2020, 06:20
EPR was invented by and is proprietary to RR. When PW made a licence agreement with RR to produce engines in USA, the EPR technology was included.
hence why GE does not use EPR.

This may be part of the historical reason in the first place - however as a patent is only good for 20 years this would not have been a cause for the non usage of EPR by GE for the last 40+ years.

lurkio
20th Jul 2020, 08:39
I was told it was because GE started with electric motors and N1 is the measurement used there. Go with what you know.

Pugilistic Animus
20th Jul 2020, 16:48
This is an extremely good thread, I learned a lot... especially thanks to TdRacer
People are probably wondering why I care as I no longer fly any jets...the answer is because I just like learning about airplanes (and to some extent) Helicopters :)

Fursty Ferret
20th Jul 2020, 17:01
TPR is slightly more useful because it works on a scale of roughly 0-100. The only benefit of either EPR or TPR over straight N1 is that they correlate directly to thrust. Two engines producing the same EPR/TPR are matched in power even though their N1 speeds might differ due to wear or damage.

Ultimately I think it's pointless and would be far happier if all manufacturers adopted the Airbus metric of percentage thrust. 100% = everything the engine can give.

Pugilistic Animus
20th Jul 2020, 17:05
TPR is slightly more useful because it works on a scale of roughly 0-100. The only benefit of either EPR or TPR over straight N1 is that they correlate directly to thrust. Two engines producing the same EPR/TPR are matched in power even though their N1 speeds might differ due to wear or damage.

Ultimately I think it's pointless and would be far happier if all manufacturers adopted the Airbus metric of percentage thrust. 100% = everything the engine can give.
​​​​​​ Just one question, what is TPR? I have never come across that term before.

Check Airman
20th Jul 2020, 18:45
​​​​​​ Just one question, what is TPR? I have never come across that term before.
The A350 (and 787 I think) have TPR as primary. It displays engine output as % thrust, so full power with bleed and anti ice off will be 100. I wanted Tdracer’s take on any engineering advantages or disadvantages to this system. Obviously, from a crew perspective, it’s a lot more useful.

Fursty Ferret
20th Jul 2020, 19:36
From RR:

The Trent 1000 engine uses TPR as the normal (NORM) parameter for thrust control. TPR is a measure of the power available at the LP turbine, used to drive the fan, which provides most of the thrust of the engine.

TPR=(P30 × √EGT)/(P20 × √T20)

P30 is the HPC delivery pressure
EGT is the Exhaust Gas Temperature
P20 is the engine air intake pressure
T20 is the engine air intake temperature

TPR will typically operate over an approximate range from 5 at idle to 100 at higher thrust levels. At a given flight condition it is directly proportional to thrust.

The 100 isn’t a hard limit, climb TPR for a cruise climb is about 106.

tdracer
20th Jul 2020, 21:15
I have no first hand experience with TPR - but it would have similar issues as EPR - namely it requires high integrity measurement of the inlet and compressor exit pressures - plus inlet and exhaust temps. Now, compressor exit pressure is a major engine control parameter anyway (the fuel flow schedule is directly tied to the compressor exit pressure) and it's hot enough that probe icing is not a concern - but it can still fail (faulty P30 is a fairly common cause of shutdowns and loss of thrust control events - some FADECs can synthesize P30 in a 'get home' mode if it's detected as failed). And inlet pressure (P20) is always going to be problematic due to probe icing and other types of probe contamination. So, you're still going to need a backup way of setting power when one of the measurements is corrupted (along with rather sophisticated error detection logic to validate the inputs) - and that backup is going to be N1.

I had some friends several years ago who were working on a 'generic' thrust setting parameter - something that went from zero (idle) to 100 (max rated thrust). The big problem they came up with was the failure modes - you'd still need some sort of backup power setting (probably N1) - so you'll always end up with two sets of power setting charts. One for normal operation, and another for backup. So long as you still need the backup, it takes away much of the advantage.

WillowRun 6-3
20th Jul 2020, 23:43
SLF here, and probably worse for a Tech Log thread - attorney, expanding practice and career into aviation (public and private international air law, specifically).

I recall the Air Florida Washington D.C. accident very well; rare is the accident without drama, yet this one was especially poignant. How ironic, or something of a similar nature, that an event nearly 40 years ago could hold relevance for understanding present-day information relating to engine operation. This might be the most important thread I've read on the site, all included. (Yes I looked up EPR and N1, not trusting intuitive guesses.)

Why? Because one of my projects, so to speak, is to try to take the curriculum in programs like the LL.M. in Air and Space Law at a certain Canadian university known for its proximity to ICAO, IATA, CANSO, IFALPA and others in Montreal, and get it back to having a strong, solid connection to aviation fundamentals. Maybe not aviating (navigating, communicating) as such, but so much of the legal profession which does stuff with heavy, even profound influence and/or impact on how aviation actually operates - so much of the profession has no idea what EPR and N1 are about. No idea how well Air Florida 90 on that snowy day in Washington illustrates how fundamental this stuff really is.

tdracer
21st Jul 2020, 00:05
Oh, one other thing I failed to mention as a plus for EPR. The EPR relationship with thrust is fairly linear - so during takeoff 1.3 EPR would pretty close to half the thrust of 1.6 EPR. At altitude, idle EPR can be significantly less than 1.0 (I've seen as low as 0.7) so the EPR relationship with thrust is less intuitive, but it's still fairly linear.
OTOH, N1 is highly non-linear with thrust below around 85% N1 - and while at high N1 the relationship is more linear, it's not 1 to 1. The number I remember was about 2.5% thrust per 1% N1 between ~90% and 100% N1.

dixi188
21st Jul 2020, 06:12
IIRC the RR Spey -511 and -512 on the BAC 1-11 used EPR, but the gauges were calibrated in % and an index number was set to correct for ambient temp and pressure so that T.O. thrust indication was always 100%.
I think the early Spey -506 had a P7 gauge, but I can't remember the indications.

Check Airman
21st Jul 2020, 06:12
I have no first hand experience with TPR - but it would have similar issues as EPR - namely it requires high integrity measurement of the inlet and compressor exit pressures - plus inlet and exhaust temps. Now, compressor exit pressure is a major engine control parameter anyway (the fuel flow schedule is directly tied to the compressor exit pressure) and it's hot enough that probe icing is not a concern - but it can still fail (faulty P30 is a fairly common cause of shutdowns and loss of thrust control events - some FADECs can synthesize P30 in a 'get home' mode if it's detected as failed). And inlet pressure (P20) is always going to be problematic due to probe icing and other types of probe contamination. So, you're still going to need a backup way of setting power when one of the measurements is corrupted (along with rather sophisticated error detection logic to validate the inputs) - and that backup is going to be N1.

I had some friends several years ago who were working on a 'generic' thrust setting parameter - something that went from zero (idle) to 100 (max rated thrust). The big problem they came up with was the failure modes - you'd still need some sort of backup power setting (probably N1) - so you'll always end up with two sets of power setting charts. One for normal operation, and another for backup. So long as you still need the backup, it takes away much of the advantage.

Thanks. I appreciate that TPR will be a derived value, and thus we’ll need a directly measured value as a standby. But from an operational point of view, EPR is perhaps the least useful gauge in the cockpit.

Check Airman
21st Jul 2020, 06:18
Oh, one other thing I failed to mention as a plus for EPR. The EPR relationship with thrust is fairly linear - so during takeoff 1.3 EPR would pretty close to half the thrust of 1.6 EPR. At altitude, idle EPR can be significantly less than 1.0 (I've seen as low as 0.7) so the EPR relationship with thrust is less intuitive, but it's still fairly linear.
OTOH, N1 is highly non-linear with thrust below around 85% N1 - and while at high N1 the relationship is more linear, it's not 1 to 1. The number I remember was about 2.5% thrust per 1% N1 between ~90% and 100% N1.

This is the problem I have with EPR being used as a primary instrument. Unless my math is bad, how’s 1.3 half of 1.6?

At best, I can give you that at idle, the EPR is ~1, but the only way I can get 1.3 being half the thrust is if full power is 2.0, which is quite often not the case. Sometimes max is 1.7, maybe 1.8. Who knows?

If you say that’s linear, I believe you, but that’s engineer-linear. Not pilot-linear :E

Thanks as always for your insight.

wiggy
21st Jul 2020, 10:07
Unless my math is bad, how’s 1.3 half of 1.6?

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.

dixi188
21st Jul 2020, 10:09
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
21st Jul 2020, 12:06
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
21st Jul 2020, 15:53
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
21st Jul 2020, 17:43
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
21st Jul 2020, 21:56
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
22nd Jul 2020, 03:37
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
22nd Jul 2020, 07:21
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
22nd Jul 2020, 08:28
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
22nd Jul 2020, 10:54
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
22nd Jul 2020, 11:33
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
22nd Jul 2020, 18:38
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
22nd Jul 2020, 21:45
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
23rd Jul 2020, 03:40
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
23rd Jul 2020, 05:27
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
23rd Jul 2020, 09:28
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
23rd Jul 2020, 23:25
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
24th Jul 2020, 00:12
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
24th Jul 2020, 04:30
<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
24th Jul 2020, 08:50
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
25th Jul 2020, 08:57
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.

MarkerInbound
28th Jul 2020, 18:04
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.

No way to know what he was looking at but the F/O knew something didn’t look right.15:59:51 CAM-1 It's spooled. Really cold here, real cold.

15:59:58 CAM-2 God, look at that thing. That don't seem right, does it? Ah, that's not right.

16:00:09 CAM-1 Yes it is, there's eighty.

16:00:10 CAM-2 Naw, I don't think that's right. Ah, maybe it is.

16:00:21 CAM-1 Hundred and twenty.

16:00:23 CAM-2 I don't know.

16:00:31 CAM-1 V1 (https://en.m.wikipedia.org/wiki/V1_speed#V1). Easy, V2 (https://en.m.wikipedia.org/wiki/V2_speed#V2).

Pugilistic Animus
3rd Aug 2020, 17:37
Just out of curiosity, who was the PF?

Twitter
4th Aug 2020, 06:49
The JT8-D 209-219 engines on the MD-80 series had a synchroniser.
Positions were Off, N1 and N2.
This was a big help in reducing harmonic beat frequency especially in the tail section, where the proximity of both engines was a problem.
Usually the N1 position provided the better result.

stilton
4th Aug 2020, 10:23
Just out of curiosity, who was the PF?


It was the Captain

Private jet
5th Aug 2020, 14:19
EPR cross check with N1. That can be a bit "iffy" because it doesn't vary much in % indication for changes at high thrust settings, although a relatively big shortfall will be obvious. Best use fuel flow too. Any jet engine design, for a given intake air density (pressure & temp & ram rise) will give a thrust output directly proportional to the fuel supplied to it, (fuel/ air ratio) so know what number to normally expect, and if you get something like that with your primary parameter set, then you are getting the thrust.
Using N1 only engines it's the only parameter you can confidently use for a cross check, but it works.
With a bit of experience, when not using autothrottle, you can set certain fuel flows to get thrust appropriate for a particular phase of flight too, final approach for example.

Pugilistic Animus
5th Aug 2020, 18:07
That's now 3 votes for fuel flow crosscheck! :}

Private jet
5th Aug 2020, 19:47
That's now 3 votes for fuel flow crosscheck! :}

With a great degree of respect sir it's not a matter of votes. This isn't a political justification of individuals self interest that occurs in Jetblast. It's the fact of the matter, it's science, engineering, operational reality. That's it.

Pugilistic Animus
9th Aug 2020, 17:06
Private Jet... When I put a :} that means that the statement is a joke and not to be taken seriously.