engine performance out of tolerances

[ATN]

Hi, everybody,

Following a client oil company auditor request, a power assurance check was performed on a 365 N. One engine has been found unsatisfactory.
The check has been performed several times and the results crosschecked and counterchecked before pronouncing the engine U/S.
Surprisingly, the technical manager flatly refused to change the engine, on which grounds, we do not know.
Instead it has been decided to limit the max gross weight to 3600 kg so as to comply with the JAR OPS 3 OEI performance level.
My fellows and I could not find anything in the books prohibiting or allowing this.
Some think that as long as the minimum RoCs are obtained it is OK. Others think that an aircraft should not fly with an engine out of tolerances, public transport or not, take off mass limited or not.
Everybody considered changing the tech. manager.

Any advice/comment greatly appreciated. Thanks.

ATN

[Genghis the Engineer]

Certainly in the UK, performance write-downs are not uncommon when an engine (or more to the point, powerplant:airframe combination) is out of tolerance but no fault can be found which is likely to degrade safety (beyond the loss of performance), and the performance still remains within certification requirements.

Given that in the UK it's generally CAA Flight Test department at Gatwick who issue these, you might be best contacting DGAC's flight test department for guidance.

Having said that, if you've got (presumably) a single rotor multi-engine helicopter, then with two different engine power output values (I assume that you're not de-rating the other engine to match) you may well be putting unacceptable internal loads or vibration characteristics onto the gearbox. In your position I'd want a clear written assurance (usually called a "Letter of no technical objection") from the TC holder that this was acceptable before accepting a performance write-down based upon a degraded single engine.

G

[donut king]

Hi ATN!

Just to confirm, are you saying the engine in question did not meet a 0 or better spec. as per the flight manual OR it did not meet the oil company contractual spec.?

Our S76A models must meet a +5 engine spec....CONTRACTUALLY! However, the flight manual requires a 0 spec. for serviceability.

Any responses welcome!

D.K

[ATN]

HI, DK,

There is no specific contractual requirement from the oil company. The engine is out of tolerances as per flight and maintenance manuals.

Cheers

[Shawn Coyle]

First thing I'd do is ask for someone to get a reading from the insurance company.
If you are not meeting the power from the engine, which is required to meet the performance in the flight manual, you may be violating the certificate of airworthiness. Since I don't know the country you're in, it would be wise to ask if someone has obtained written confirmation from the insurance company that they are happy for you to continue with this.
The next part depends on whether you are operating to Category A or not. At least one helicopter I know of has a limitation that you must have passed the power check to operate in Category A. Check your flight manual, and if it has words like that, you can't operate to Category A.
Making a decision on performance like this is not a simple one, and I would have hoped that some advice from the airworthiness section of your civil aviation authority was made aware of this, not just the operations people. The implications for airworthiness are pretty major. An arbitrary reduction in weight may not take into account the significant reduction in engine power available that occurs in warm temperatures, for example.

[dammyneckhurts]

"The engine is out of tolerances as per flight and maintenance manuals."


No multi engine experience here...but I cant even imagine a Tech. manager that refuses to change and engine that is not up to spec. (assuming that the torque/ temp/compressor speed indication systems have been checked?)

Seems that it would be much safer to figure out the problem in the shop....in this day and age of litigation I would have to wonder if in a court of law a pilot would be held responsible for flying with an engine that is know to be less than spec.

Seems to me that you need two things....a new engine (at least till the problem is isolated), and a new Tech. manager!

[donut king]

In this case, engine performing below flight manual and maint. manual spec., I would GROUND the pig!!

As others have stated..... C of A. is invalid AND the PIC is liable for flying an u/s aircraft.

Lawyers will have a field day on this one!

D.K

P.S. ATN, brother, protect yourself first!!!!! (ground the pig!!!)

[LPS500]

From experience with the Turbomeca engines (Arrius - the Arriel NEVER fail), be very careful with the OAT when doing engine condition checks. We've found that plotting the graphs using the aircraft OAT gauge will often fail the engine. This gauge is not calibrated normally so could be well off. We take a calibrated digital temp gauge along for comparison, which was suggested to us by Turbomeca. Another thing with the Eurocopter aircraft is that if you fly low for the condition checks, say up to 1500ft the engines will often fail, try flying 3-4000 and see what happens - best to be in the fat part of the graph rather than at the low end. As far as performance height climbs go the CAA have some interesting rules if anyone would care to know...........

[NickLappos]

For Part 29 helicopters, the power assurance check is used to measure the engine against a standard, and to therefore assure the airworthiness of the helicopter. The performance is not necessarily assured if the engines are not up to spec, but also the cause of the low power might be a condition that affects the engine's airworthiness, in that it could be precourser to an impending failure.

Generally, consistent below spec power is reason to ground the helicopter.

Some comments:

1) Marginal engines, quite fit but right at spec, will by the forces of nature measure below power half the time and above the other half. This is the normal statistical fallout of any measurement, and not a sign of sickness. Some machines are certified with a daily averaging method to allow reasonable assurance even if a particular check is low, as long as the rolling average is acceptable, and there is no obvious other problem. I think a zero margin engine can be operated in the face of it failing half its checks, as long as documentation like a recorded history supports the reasonable assumption that it is simply borderline.

2) The OAT/FAT gage is a prime contributer to this scatter, since turbine engine power varies by about 1% power for each degree C. That means that one degree of variability on the gage could spell doom for an otherwise healthy engine. The typical bi-metalic temp gage is not really designed to provide the accuracy we ask of it.

3) the pilot procedures during the check are critical, and any sloppy handling or hasty reading could make acceptable aircraft seem unhealthy. The biggest problem is facing squarely into the wind, so no reingestion of hot exhaust occurs. One degree of OAT rise is about 4 degrees of engine temp, so a little reingestion is a bad thing. Checks done on the ground have more scatter, and the possibility of more hot ramp air finding the inlet than fnding the OAT gage, so many helos have in flight power assurance procedures, where clean air and steadier conditions help reduce the scatter.

4) Below margin engines might otherwise be quite safe, just a bit more eroded or dirty. Sometimes, engine manufacturers file letters and maintenance procedures to allow below spec operation, as long as no other condition is at fault. This allows the pilot the assurance of continued engine safety, but not adequate aircraft performance. For below spec operations to be legal, there must be flight manual performance procedures that allow the pilot to determine the flight performance with the amount of degredation folded in. This is sometimes done by providing a rule on the chart, such as "For engines below spec by x percent, enter the chart 900 feet higher than ambiet for each percent below spec" Thus the normal chart allows the pilot to precisely calculate. In US Military operations, this is quite normal, and up to 10% power loss is not a grounding condition (15% in the case of the CH-53E!)

4) The situation can be reversed, so that the pilot could be provided with above spec charts to take advantage of more than spec power.

5) If the failing power assurance is a relatively sudden drop, say more than 2 or 3 % in a day, it is almost surely a sign of a maintenance problem. Salt encrustation, bleed valve leaks, dirty engines are often the cause. Sometimes, an internal engine crack could be allowing hot gasses to be strangely circulated, and the drop in power could be early warning of a catastrophic condition just about to happen.

[ATN]

Thanks to all for the answers.

NickLappos

The checks are performed in cruise flight.

To remain within the JAR OPS 3 requirements Performance Class 2 - 150 ft/mn @ 30 mn power rating - the T/O mass has been reduced from 4000 kg to 3600 kg.
The difference with the specs and the second engine is significant so the impact of the OAT gauge is not the factor here.

The manufacturer does allow a slight drop of performances due to erosion or dirt - this is what tolerances are meant for IMO, but only up to a point.

"One degree of OAT rise is about 4 degrees of engine temp"

Is this a general rule and is it linear ?

Genghis

I can't understand how an engine can be out of tolerance and still be within performance certification. I believe if an engine is declared out of tolerance that's precisely because it cannot provide the certification performance level, or is it just a matter of wording ?


Thanks again

ATN

[Vfrpilotpb]

Consider this,

What happens if the good engine packs up, will the other give you enough for complete safety to fly out of trouble,

and, what if this lack of performance is a very small sign of something that is very slowly going pear shaped.

Safety is of the most importance, engines are expensive, but litigation and compensation can be prohibitive for the future good of your company!

Change the donkey!

Peter R-B

[Genghis the Engineer]

I can't understand how an engine can be out of tolerance and still be within performance certification. I believe if an engine is declared out of tolerance that's precisely because it cannot provide the certification performance level, or is it just a matter of wording ?

There are two separate issues here.

(1) Does it meet the absolute minima of the safety standards.
(2) Does it provide the performance values in the published operating data.

If the answer to (1) is "No", then clearly it's totally unacceptable. If the answer to (1) is "yes", but to (2) is "No", that's when a performance writedown is used.


A classic example here is the Harrier (okay, not a helicopter, but it does hover), where the deck-launch limits and hover performance limits are incredibly tight. There is actually a formal scheme whereby every limit is effectively written up or down from standard, and all the operating data reflects that. Clearly there are threshold limits at which the engine is rejected, and continuous monitoring is needed - but it's just an extreme example of the same thing.

G

[NickLappos]

ATN,

1) 1 degree of OAT drives up the operating temp of the engine about 3.5 degrees, based on the thermodynamics of the way an engine works. It is mostly linear, but slightly different for some engines. Look at the power assurance chart (which might show target T5 vs Torque), and simply enter it at the same altitude, same power, but at two different OAT's. The ratio of OAT to T5 is usually about 3.5 or so.

2) The answer Ghengis gives is that the performance of an engine can go below spec, but the engine might not dangerous to operate. That is what "airworthy" means. Most engines can degrade below spec cheerfully, to perhaps -10%, before they get non-airworthy. If the cause of the power loss is erosion or dirt, and not busted parts inside, the engine will probably degrade further until it can't accelerate properly, and it starts to stall when you ask for power increases. That is because those raggedy compressor blades no longer smoothly transport the air to the combuster, so the engine flow becomes unstable. Most degraded engines worry the engine manufacturer because of this possibility of stalling on acceleration, and that stall sets the minimum safety point. I must say, though, that most engines have no such published permission to operate below the min spec point. When performance is a problem, the aircraft manufacturer and the engine manufacturer collaborate and get the airworthiness authority (CAA, FAA, or the military service who bought the machine) to agree. They agree by publishing such a procedure in the flight manual and the maintenance procedures. Only then is this theory we describe allowable.

3) when you say the gross weight was reduced in the aircraft in question, from 4000kg to 3600kg, was this done using a factory (flight manual) procedure? If so, it is the kind of "below speck" procedure I discussed above. If it is an ad hoc procedure made up by somebody else, be wary. only the regulatory agency who approves the flight manual has the ability to wave such limits.

[ATN]

Nick,

Thank you again for your input.

I think we are saying the same thing only using different words.

The weight reduction has been done using the RFM perf. curves but the procedure you describe does not exist for this type.

We are convinced we reached the limit between what can be done and the unacceptable.

We are surprised that the oil company aviation experts have accepted this compromise, or we must be missing something.

Cheers

ATN

[HOSS 1]

You say the chief mechanic refuses to remove the engine? This sounds very strange to me. I would think it's a simple as following the published procedures in the engine and/or airframe maintenance manuals. It's an If-Then type logic. If it fails, perform engine wash and try again. If it still fails, remove for servicing. Plain and simple.

Based on our engineering discussions with Turbomeca, their engines are NOT certified for below spec power operations. As a matter of fact, only just recently were they asked it be added to an engine TC (Arriel 2S2). They told us that currently the DGAC will ONLY approve below spec operations on a case-by-case basis, and then ONLY so aircraft could be ferried to a maintenance facility.


As an engineer, I would want written approval from the engine AND airframe manufacturer clearly approving a flight (WAT) envelope. Including all limitations. (Good luck getting this!)

As a pilot, I would insist on some sort of paper from the cognizant local regulatory agency clearly allowing this operation. A iron clad CYA, if you will. (And even MORE good luck on getting a gov't employee to risk his neck on this one!)

Ask this Maintenance person for the approval paper trail. After all, it's your rear in the seat!

HOSS-1

[NickLappos]

Nicely said Hoss1. I fully agree.

[soggyboxers]

I do wish some posters would read what you said before posting their replies!
However, I have to say that I have found it to be fairly unusual for a 365N to fail its S/E height climbs. If it does it's usually a sign that there is a problem with the engine or one of the parameters. To eliminate one of the most common errors, it's a good idea to use a calibrated digital OAT guage for the height climbs, and then to circle at the mean height for about one minute to get an accurate temperature reading for that height. I also find it helps to carry out the flight early in the morning or in the evening to negate errors due to turbulence (a flight over water also helps in this respect, where possible). I try to start at a height which keeps me out of turbulent air and climb for 5 minutes to get good 'averaging'. When we have had engines which have (rarely) failed we have always then looked at the factors which may have caused an erroneous result - inaccurate OAT readings, inaccurate Ng and Nr gauges, incorrect basic weight and/or fuel calibration, 'air' leaks (P2 air, or heating/vent ducts which are leaking). I carry out ground and in-flight 'power assurance' checks iaw the FM as a cross check and look at the average rates of climb throughout the flight to see if over any 30 second period they have deviated from the 'graph' due to turbulence or my inaccurate handling. If everything has been double-checked and the aircraft fails there is only one thing to do with a 365N - ground it and wait for the new engine to arrive. There is no consideration given in the FM (as with some models of the S76, for example) for a reduction in MAUM for Cat A operations for below spec engines.
If I were in your position I would put the aircraft unserviceable in the Technical Log and if your Tech Manager refuses to accept that have a word with higher management in your company as to whether they value the services of yourself or the Tech manager more! His attitude is totally unacceptable if you are operating for an Air Transport Operation in the 21st century, and would never be condoned in my company.

[ATN]

HOSS 1,

This is exactly what my colleagues are going to do - I am off at this time.

Why in the first place did the technical manager deny this engine change is beyond our understanding as well as the approval from the oil company aviation experts. It's all the more strange that the company is on an hourly basis insurance system for all the main components, including engines

What does WAT stand for ?

ATN

Soggyboxers,

The procedure you describe is to be used when the result falls very close to the dividing line, which is not the case here since the engine is well below specs.

"There is no consideration given in the FM (as with some models of the S76, for example) for a reduction in MAUM for Cat A operations for below spec engines."

This the the core of our argumentation.

[HOSS 1]

WAT = the Weight Altitude Temperature curve(s). Use any two to get the third.



HOSS

[offshoreigor]

ATN,

I have NO time on your type but quite a bit on Turbomeca, specifically Ariel 1S1. On that engine, the power assurance is not a 'go no go' figure. Does the engine pass the ICP and 2 1/2 minute power check? Does your company perform "Total Performance Checks"? This check is done at Max Continuous OEI to measure climb performance against the RFM graphed performance, but I'm sure you've done that. I'm just curious as always.

Cheers, :eek: OffshoreIgor :eek:

[ATN]

Offshoreigor,

Everything has been done except the ICP check. I guess the 1S1 is fitted on the S76 A + or A ++ - I have not flown the beast for ages. From the previous posts I understand that for some types the RFM provides a procedure where the MAUM can be reduced to make up for engine below specs. This may be the case for these S76 versions but not for the 365N, so the power check may be a go no go figure.

Cheers

ATN

[sandy helmet]

Does anyone know if this reduction in MAUW procedure for marginal engines apply to Bell 212/412? (PT6-3 and 3B).

[HOSS 1]

OffShoreIgor-

Which Sikorsky model(s) allows continued flight after failing power assurance? I know the CAA cert'ed Allison powered 76 does, but am not aware of any turbomeca powered 76's.

Please help me understand this !!!

Which model(A+,C,??) and who's approved the RFM (jaa, caa, faa???)


HOSS 1

[offshoreigor]

HOSS 1

The daily power assurance on the S76 is not a go no go, period. That applies to all models, whether it be A, A+ or A++. The ICP is the go no go and on the A++, the 2 1/2 min power check and the total performance check.

This makes sense as the 2 1/2 min check tells you the engine will fly you away if you meet the OEI SSE speed and the TPC tells you that you will continue to fly, OEI at Max Continuous OEI at VBroc.

Hope that answers your question.

Cheers :eek: OffshoreIgor :eek:

[HOSS 1]

It's my understanding that on the C+, failing a single point power assurance (which must be done once per 20hrs) IS a no-go. As stated in the RFM. Maintenance must be performed.


Thanks for your replies.



Thanks !!!!

HOSS 1

[edited to remove my obvious confusion. Questions already answered]

[donut king]

Hoss 1,

The 76's daily power check Offshoreigor is referring to is just a trending check. It must be coupled with a 50hr ICP which is the actual go/ no go check.

If that helps!!!

D.K

[spinwing]

MMmmm very interesting but back to the original question ....

"why did the Tech Mngr refuse maintenance action"? and give NO REASON!

If the failure to reach minimum power specification was written up in the A/C tech log or maintenance release THEN it effectively grounds the aircraft untill it is cleared by a suitably licenced engineer/mechanic. In clearing the defect he will have to justify in what way that defect is allowable IAW the responsible Regulatory Authority, or the Approved Aircraft Maintenance documentation or procedures.

If as a pilot you are not happy with that then do not accept the A/C and involve your Chief Pilot.

BUT DO CYA (Cover Your Arse) if it turns to **** the paper trail will hopefully absolve you and allow your kin to sue the Tech Manager and the Company that resolved to perhaps direct (force?) you to fly an A/C of questionable serviceability.

[ATN]

Spinwing,

I am off - I left the day the check was performed, so I do not know the Tech Mgr arguments - but sure must have some.

Offshoreigor,

We do a daily trend check, we were talking about power assurance check and THAT is a go no go.

Cheers

ATN

[offshoreigor]

ATN,

Which Power assurance check do you do? We also do a daily Power Assurance "Trend Check" at 96% N1 @ 2000' PA. But the real go no go is the 50 hr ICP.

Cheers, :eek: OffshoreIgor :eek: