The Old N1 and EPR again!
 

To all those drivers of engines with BOTH N1 and EPR guages - do you have charts to give minimum N1 on T/O for a given EPR and OAT??
If not - why not??

I am not in the same place as my manuals at present. However on the RR535-E4 thrust is normanlly set on the EPR. If the EPR guage was u/s then there is a chart to give you the required N1 against Pressure Altitude and Temperature.

If you are concerned about faulty EPR then this would be found by mismatching parameters between the two engines

Thanks SAM 2M - I drive a couple of BMW-RR 715's attached to a 717 - no charts for Min N1 or N1 if EPR fails - it seems strange to me that this has been allowed to get thru as I have no way of crosschecking that the power required is being attained.

Any other RR drivers out there that wish to comment - have bought it up with RR & Boeing - no reply yet.

Remeber that 737 that had ice on its engine probes and went into the Plotomac (spelling??).
Those guys set the required EPR but due to faulty indications didn't have anywhere near the required thrust.
For this reason its always a good idea to know what sort of N1 indication you'll get for a given EPR and temperature, so you can tell if there is a problem.
Cross-checking engines is good, but remember if one engine is indicating wrong there's every chance that the other one will be doing the same.
On A/C I fly we use JT9D and RB211, the JT9D has a nice chart (marked guidance only) called "Nominal N1%RPM Take Off". The RB211 only has a chart for Go-Around N1% in alternate EEC mode, but I guess it can still give you an idea.

I fly with the JT9D and we always put both T.O. EPR and N1 on the bug card. In addition we call "Thrust set, N1 checked" when T.O. thrust is set.

It is not difficult to calculate the N1 setting, if you already know the limit N1 and the flat-rating temperature.

For example, if a certain high-bypass fan-jet is flat-rated to 32°C at 98.0% N1, then you could determine the limit N1 at any LOWER temperature. The conversion factor is : square root of the ratio of OAT and flat-rating temperature (all temperatures must be in Kelvin).
eg. at 10°C, 98.0% x sqrt((273+10)/(273+32))
= 98.0 x 0.96326
= 94.4% is the limiting N1 (max T/O power)

For deration, example OAT is 15°C and flex temperature is 45°C: if the limit N1 at 45°C is 93.3% N1, then at 15° the N1 should be 93.3% x sqrt((273+15)/(273+45)) = 93.3% x 0.95166 = 88.8% N1
The flex N1 is 88.8% at 15°C (flex temp is 45°C)

You could make a spreadsheet showing the derated N1 against OAT, and take it with you for flight. Use it to cross check the N1 you get on the engine instruments (if your engine is EPR rated). But remember to make separate charts for different pressure altitudes (sea-level, 1000', 2000', etc)

[This message has been edited by Old Dog (edited 20 November 2000).]

Ask one of your guys with a SITA telex connection to contact SEABO7X (Boeing). They should be able to answer.

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rgds Rat

Sounds like something you'd get the Second Officer to work out Old Dog :)

HPSOV, you'll be surprised to learn how many people could determine the correct N1 without a tabulated chart or the FMC. (Very few!)

If the learned and experienced Captains could not, the young and inexperienced effohs won't improve on the statistics very much.

If you care enough, it is best to prepare your own charts, if your company did not provide any.

I might have missed the point of the post, but if you are looking for something to back up your EPR, why not fuel flow?

Or are you looking for an N1 to use with a failed EPR? Will your MEL allow you to dispatch with that?

After all, power is proportional to fuel flow.

You can actually ballpark some aircraft manouevres via fuel flow for the power part of the power + attitude = performance.

Works for piston as well as gas turbine.

In theory I guess, but all engines are not equal, they get tired, dirty, worn, out of trim etc, also when you select a a/c pack on say, the fuel flow increases and the egt increases, but the power is still the same!

757 RB211 E4's

EPR is the main thrust parameter, however we carry in our QRH performance tables based on OAT and thrust..as previously described.

One of the early DC-10 customers was National A/L, and their engineering was very skeptical of N1 power mgmt. so they insisted on EPR. (All other CF6 operators accepted N1 after some convincing...)

But after Pan Am bought them out, they reconfigured the gages to agree with DC-10's in the rest of the world.

Beware the EPR readings in the 737-200. We once had blocked Pt2 inlet reading on both engines due foreign objects finding their way into the sensor tubes. OAT 30C. Obviously not ice. On take off both EPR digital read-outs were equal at 2.18 EPR but somewhere along the take off run it was apparent that the acceleration was not normal for the indicated EPR.

In the darkened cockpit and with a very short runway and as all needles were identical on both engines, no one took a long hard look at the N1 indications. When it was obvious that we were never going to get airborne before the end of the runway, the captain firewalled both engines, rotated quickly, and got airborne at Vr minus 10 knots. On firewalling, both EPR leapt to around 2.27 (max allowable 2.18). Investigation revealed that in all probability, the actual N1 during the take off run was around 89% when for the conditions that night it should have been 101% N1 for 2.18EPR.

Which means of course the N1 is different as well. Quite common to see two unmatched engines producing the same EPR but having to use more N1, higher EGT and more fuel to do it. With degradtion of fan, turbine and compressor clearances etc. the engine has to go round faster to produce the same thrust.

Barit1,

National were not the only ones with EPR on the DC10. SAS also had EPR on theirs, although N1 was the primary parameter.

Might have been the same on other KSSU DC10's as well.

Fuel flow would be the last thing I'd use to cross check a required EPR. F/F is accurate ONLY as it leaves the transmitter. Crack a line between the F/F TX and the nozzles and it may well be the red light and bell that tells you why the EPR has a shortfall.

All the KSSU DC-10's (vertical scale gages) had a common cockpit including EPR IIRC. But EPR was often neglected because it had no operational use.

ATLAS ships had steam gages, with N1 at the top of the engine stack and EPR at the bottom (if it was installed at all...). National was unique for CF6-powered DC-10's in that EPR was used for power mgmt. and was at the top of the stack.

Can't speak for DC-10-40's, but with JT9's I'm sure they were pwr. managed by EPR...

I seem to remember that on the KSSU DC10's the EPR was measure between the pitot probe and the 5.4 station, there was no Pt2 probe. So it wasn't a 'real' EPR anyway.

I believe it was mostly used for condition monitoring. As the 5.4 pressure still is on some ships.

You are right about the -40. Dunno why anybody would choose the JT9-59A over the -50C. Sure it is more solid, but then it also needs to be. And it needs a lot more 'fixes' for airflow control in the compressor.

Aircraft with both EPR and N1, the EPR is used for takeoff setting. In the event of EPR being inoperative, N1 is used for primary thrust setting. Takeoff N1 can be obtained from Aircraft Operations Manual volume 2. If however, a EPR is considered inoperative there may be a large weight penality incurred.