I was wondering if any pilots/engineers who have experience with the above mentioned could answer a few questions for me?
From a pilots point of view, for those who have experience of both engine types on the A320 family, which engine performs better?
For the engineers, when I've seen the V25's and the CFM's being kitted with the powerplant material before returning to their respective airline, the CFM looks a far easier engine to work on - so.....can anyone give a broad outline as to which engine is more reliable through personal experiences
And finally, if anyone can answer this - operationally, which is the preferred engine type to those who have access to both of them?
7500 on A320 family, 75% CFM, 25% V2500
No preference really, both have their advantages/disadvantages.
CFM, quicker starting, lower idle thrust,higher reliability BUT, higher fuel and oil consumption.
V2500, more fuel/oil efficient, more effective reverse thrust BUT slow starting, high idle (more braking during taxi) and seemingly more problematical.
IAE is undershooting speed when levelling off from above, generally reacts very slow compared to CFM. The thrust indication is sometimes very hard to recognize, because the EPR number between idle and TOGA thrust isn't much different. A lot of pilots use still N1 or even fuel flow to cross check.
Its a while since I worked on either of them, but as an engineer I'd rather look after a fleet of CFM powered A320s than V2500 powered.
I just like an easy life and can do without having to open the engines every other day to troubleshoot some spurious (usually pneumatic) fault.
We looked at both, from a route/payload/fuel burn perspective, the IAE was better, but historically we have had big problems with the V2500 on another fleet type, so we picked the CFM.
We operate A319s/A320s with the CFM-Engine and A321s with the IAE-Engine.
As a driver, I'd prefer the CFM any day. The problems have already been named, the greatest disadvantage ist the mickey-mouse-style thrust indication on the IAE.
If you have a grossweight (landing weight) of lets say 58 tonns on a CFM-A/C, you would set N1 to GW-3 as a thrust target xx% N1 during approach, in this case some 55% N1.
On the IAE your target ist 1.0xx EPR and already by looking sharply at the trust levers you change EPR easily by > 0,01-0,02. So good luck in a heavy A321 (greetings from the slow reacting engines) on a windy day.
Also the EPR display on the IAE-A/C is a good bit smaller behind the decimal (compared to the N1-Display on the CFM-A/C), so looking at it to read the EPR takes always just this 'microsecond' longer, in which you should have been looking at the intruments/outside.
I guess what max is talking about is a datum power setting on approach. Take the gross weight of the aircraft (58T) and subtract 3 (GW-3!) and that would be your reference N1 on approach (i.e. 55%).
Did about 10 years on CFMs, then 4 years mixed, before retiring 7 years ago. Agree with all I've read here. Have never liked EPR as a means of power indication – except perhaps for take-off – for the reasons others have highlighted. In the 'Seventies, we had similar problems on the BAC OneEleven when converting from early Speys, which used N2 as the main thrust indicator (except for take-off, when P7 was also checked), to the later Speys.
Fuel flow indication should be a useful tool but, if memory serves, is almost useless when the throttle is moving, when you may need it most.
Presumably, the N1 is as good an indication of changing thrust on the IAE as on the CFM (at a given density altitude), so CFM-sized N1 "gauges" would be the answer. The two snags that spring to mind are:
1) lack of space on the display;
2) the misguided (in my opinion) policy of most airlines to discourage or even ban the use of manual thrust, despite the A320 having the best "manual" throttle of the seven jet types I flew. Most of the once-a-month brigade would never dream of it. And the A/THR never fails, does it?
The other thing I liked better about the CFM was the comparative ease of inspection on a walk-round, particularly if you were wondering about a bird encounter on the previous sector.
Almost impossible to fly the 2500 on EPR as already explained due to small range of movement. Everyone I've spoken to flies it on N1.
Dont forget the potential inherent errors in any EPR indication (737 Potomac accident). Engine speed is the best indicator of real thrust.
Apart from that, the 2500 may be more fuel efficient but is a bit of a pain for the long start cycle, the slow response in levelling off and the EPR/N1 issue. However, they both work ans as long as they keep turning ....
We had IAE's on the fleet before, changed to new ACFT with CFM's.
Nevertheless I believe that in high altitudes the older IAE's had more power than the brand new CFM's. No data to confirm my "feelings"
- IAE - do NOT trust the -A1 variant of the V2500, it a piece of
- IAE - the -A5 is much better, but not so mechanic friendly, lots of grazed knuckles and burnt elbows
- CFM - A much better creature, lasts forever, has very little in service issues
Just to pick up on some pilot comments on the EPR, if you're only issue is the guage then consider this, from an efficiency point of view (fuel burn/thrust) EPR/thrust relationship over time is much more accurate than N1/thrust, since it is based on pressures, not rotational speed.
The relationship of N1/thrust is only as accurate as the day the engine comes off the test-cell, and the N1 trim is set. When the fan track wears out, the airfoils get dirty and the blade l/e's erode etc the N1 speed is no longer related to the thrust it was in the test cell - therefore on a high time CFM engine you may be 2% down on thrust.
So please if you're issue is with the guage - ask the aircraft designers to make a bigger display!
N1: Good point. Instead of the engine lot had so far focused on implementation. Even the Potomac reference is historical and things may have evolved over time. But I wouldn't know being CFM dependent. To bite one's lip, A319 with IAE looks way too cool, one of my favourites. Somewhat scary, think 262. Steed of the breed, imagewise.
A Pratt and Whitney engineer once told me that EPR is better because a 1% error in EPR indication is a 1% error in thrust. Using N1, a 1% error in rotational speed could be as much as a 4% error in thrust depending on actual engine speed. The Potomac crash was not caused by faulty EPR, but a whole host of other problems which caused, among other things, incorrect power being set.
Quote from N1 Vibes: The relationship of N1/thrust is only as accurate as the day the engine comes off the test-cell, and the N1 trim is set. When the fan track wears out, the airfoils get dirty and the blade l/e's erode etc the N1 speed is no longer related to the thrust it was in the test cell - therefore on a high time CFM engine you may be 2% down on thrust.
[Unquote]
Point taken. [And maybe the N1 VIB off-scale at 9.9(+) means one of the fan blades is missing anyway?]
Trouble is, if memory serves, there's something non-user-friendly about EPR readings, particularly at the medium and lower power settings; like in the hold and on instrument approaches. But I can't quite put my finger on it. This was particularly true on the JT3D and the later Speys, both of which had analogue gauges where you'd struggle to see the difference between 1.20 and 1.25 (= quite a power difference).
Perhaps someone like Mäx Reverse can remind us how many places of decimals are displayed on the presumably accurate, digital V2500 EPR display. Is it 2, or 3? If so, maybe the trick I never learned might be to knock off the "1" mentally, and concentrate on the decimals. Then come up with some rules of thumb as-per N1.
But for those who fly both engines, like BA and currently easyJet, it's going to be a challenge to remember both sets of figures. Particularly if the management frown on (or ban) the use of manual thrust anyway.
Here I go again...
That's the way the Engine/Warning Display looks for a CFM-engined Aircraft. N1 Displays (digital and analogue) in the uppermost part of the display and pretty well readable. Notice Fuel Flow Indication is in the 4th line.
For an airframe with the IAE-installation the display looks like this.
EPR indication in the first line with three mickey-mouse-style decimals. Far too small to be well readable, especially during turbulence or darkness.
N1 now in the 3rd line and N2 in the 4th, moving FF out of the way to the right, completely different place than on the CFM-Display. Hard for those setting power according to FF.
For an A321 with 'usual' LWs (ie not empty) the rule of thump is target on approach = 1.0xx where xx is the LW. 70 Tons would give you a target EPR of 1.070. No idea how this works on an A319 with 47 tons (A3192/320s are CFM-ships with us).
If you are unlucky enough to loose EPR-Mode the fun really begins. Instead of interchanging the position of the now xxxed out EPR and the N1 the blank EPR gauges stay at the top and you find yourself looking for the N1s in the 3rd line.
The V2500 display looks depressingly familiar. They should definitely enlarge the decimals to the same size as the "1", reducing the size of the latter if necessary. I'm a purist, but I still think the "1" is over-emphasised at the expense of the other digits. Correct me if I'm wrong (it's been 7 years), but by definition the EPR is never less than 1?
The fuel flow display is squeezed over to a non-ergonomic position, which is also a shame.
To sum up: room for improvement.
Was I right (Post#10) that digital fuel-flow readings go haywire during engine accel/decel? It's a pity, because fuel flows used to work brilliantly as a thrust indicator on early Speys, for example. They are pretty-well independent of ambient temps at a given altitude, and, flying on one engine, the total fuel flow is about the same (well, maybe 10% more to allow for the sideslip).
Interesting that the graphic shows the No.2 engine N1 spooling up slower than No.1's, although the EPRs are the same. But it illustrates well that the FADEC's basic job is to provide an EPR.
Last edited by Chris Scott : 7th November 2008 at 20:32.
Reason: Added: Total fuel-flow as an indication of total thrust
Correct me if I'm wrong (it's been 7 years), but by definition the EPR is never less than 1?
Not as long as the engine is producing thrust
Obviously AI has already discovered the room for improvement on their own. The Display on the newest ships with the LCD-Screens (instead of the old CRTs) looks completely different again.
As I was told several restrictions imposed by an old DMC-Standard in combinaition with the slow cursor of the CRTs made these changes impossible until the advent of the LCDs.
The IAE-Display looks like this on an LCD:
Much more spacious and the numbers are all the same size. Bravo!
However the LCD-ships take a lot of getting used to, especially if you fly mixed as EVERY page looks more or less completely different. But us lowely outstation-folks are not normally even allowed to touch them, so why do I bother? My broad experience one year after EIS: 1.0 legs!
To bite one's lip, A319 with IAE looks way too cool, one of my favourites. Somewhat scary, think 262. Steed of the breed, imagewise.
Still speaks nothing about the engine itself ...
Linear Mode
