SandBlaster-214

All:

Mr. Lappos suggested I post this situation for discussion. For myself, this came up some time back but, I was recently browsing (catching up) through the threads and saw a couple of engine limitation question and answer postings and, well..., I suppose it fits in with those.

BACKGROUND

A while ago, I had been asked to render an opinion to some folks I once flew for as to the possible detrimental effects of continually (not continuous) operating an engine above the max continuous MGT (or whatever you want to call it) rating.

The goof's (read: pilot) "normal" operational cruise configuration is (was) pulling into the top of the five minute MGT limit for four minutes - thirtysome seconds then reducing power to max continuous MGT for fifteen to thirty seconds and back up again. Unfortunately (for him), the dude's internal timer malfunctioned, he didn't get the power down in time and the gauge recorded the exceedence. Pilot's response: "...it's done all the time..." and, "...standard practice here..." and, "...where does it say you can't...".

The Company contacted the manufacturers whose response(s) to them was: [in essence] "Refer to flight manual".

Now, lets go a step further here by establishing some constants... Company operates several of the same make/ model aircraft. All are assigned to a specific base within a region of the country (i.e., while some very limited intra-region swapping is done, the aircraft are base and crew specific). So, lets assume the aircraft in question is "Aircraft A". Aircraft A has a long history of turbine problems that could be attributed to heat and has yet to make TBO/ TBI on the hot end while Aircraft B, C, and D have experienced no similar problems. This is not engine specific as Aircraft A has had several different engine/ turbine configurations installed with "premature problems" on each. Crews B, C and D state they do not operate as Crew A and the onboard monitoring systems concur. Mission profile are identical and finally, if a difference does exist, assume Aircraft A operates in the least hostile altitude/ temerature environment.

For clarification, I ain't no Norman Einstien (or his brother Al). I'm a driver - nothing more. I been doing it for so long folks are of the mistaken impression I know **** so they end up axin' and I give 'em my opinion (which I did) based on the ol' "what would I do if it were my aircraft and operation" answer and leave the thinkin' up to the folks that are good at it.

By the way... Nick, the real Al Einstien, said he'd provide the real answers to the question.

Anyway..., have fun - it'll be interesting to see how FUBAR I wus.

NickLappos

SandBlaster-214,

This often comes up, and is worth a bit of explanation.

In a few words, if you push the engine at 5 minute power for 4.5 minutes, then reset to something lower, then go back to the 5 minute rating, so that you are operating nearly continuously at 5 minute power, you will be seriously damaging that engine. Your overhauls will occur much more frequently, and perhaps failures occur much more often.

That the pilots would believe that the engine "cools down" so that it can be abused again is indication that they do not understand how aircraft components are rated.

Imagine that the engine has 10,000 beans in a jar when it is built, and that these beans are withdrawn at the rate of 1 bean per hour while it is at 75% cruise power. If you cruise at 60% power, the beans might be spent at 1/2 bean per hour. At 30 minute power, the beans will be expended at 5 beans per hour. At 5 minutes take-off power, they will be consumed at 20 beans per hour. At OEI power they will be consumed at 100 beans per hour. Once the beans are gone, the engine has warped blades, shroud rubs and poor power. Its vibrations are high, and it is closer to failure. You can spend the beans as you wish, but clearly it costs you more to use the engine above its rating. These ratios are not too far off from the real ones, I think.

The engine gains no benefit from cooling down between the cycles, because the time at the cycles is doing damage, as every running minute does damage. It was George Saunders who once wrote that metal has a memory like an elephant, and it will recall everything to did to it. We call it creep, where the blades are being bent, warped and grown as they operate at elevated temperatures.

When the manufacturer rates the engine, he tests it against a assumed spectrum, which accounts for the power vs time it will operate. The spectrum is a guess on how the engine will be used, and a way to predict overhaul time.

The trick your pilots are using is unwise, and unfair to the owner of the helicopter, and also in technical violation of the limits. The 5 minute rating is a take-off rating, of course.

Does this mean that a guy who takes off every 5 minutes and uses Take Off power to do so is wrong? Nope, because he is taking off! His engines will be more harshly used, but he is legal. He would also be wise to come off of take-off power as soon as possible each time.

Remember the beans in the jar.

Nick (absolutely NOT Al Einstein!) Lappos

Gomer Pylot

Well, [expletive deleted]! Wrong again.

I thought I couldn't be amazed by the stupidity of people any more, but once again I was wrong. I'm sorely tempted to say more, but I *will* resist.

[email protected]

An interesting thread, I have lost count of the times I have been confidently told by pilots that the Maximum Contingency rating found in Military documents and typically of 2 1/2 minutes duration is available either once per flying hour or once per sortie or once per day. Nick's information would conclude that it is once only in the life of the engine and after it has been used the engine is irreparably damaged and is destined for a gradual decline in power output.
It is a question of interpretation - how long is a transient limit allowed to be used - 1 second/5 seconds/20 seconds and again how often is this transient allowed in the life of the engine?

John Bicker

Rated 30-second OEI power, with respect to rotorcraft turbine engines, means the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine under part 33 of this chapter, for continued one-flight operation after the failure of one engine in multiengine rotorcraft, limited to three periods of use no longer than 30 seconds each in any one flight, and followed by mandatory inspection and prescribed maintenance action.

Rated 2-minute OEI power, with respect to rotorcraft turbine engines, means the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine under part 33 of this chapter, for continued one-flight operation after the failure of one engine in multiengine rotorcraft, limited to three periods of use no longer than 2 minutes each in any one flight, and followed by mandatory inspection and prescribed maintenance action.

Rated continuous OEI power, with respect to rotorcraft turbine engines, means the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine under Part 33 of this chapter, and limited in use to the time required to complete the flight after the failure of one engine of a multiengine rotorcraft.

Rated 30-minute OEI power, with respect to rotorcraft turbine engines, means the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine under Part 33 of this chapter, and limited in use to a period of not more than 30 minutes after the failure of one engine of a multiengine rotorcraft.

Rated 2 1/2-minute OEI power, with respect to rotorcraft turbine engines, means the approved brake horsepower developed under static conditions at specified altitudes and temperatures within the operating limitations established for the engine under Part 33 of this chapter, and limited in use to a period of not more than 21/2 minutes after the failure of one engine of a multiengine rotorcraft.

Rated takeoff power, with respect to reciprocating, turbopropeller, and turboshaft engine type certification, means the approved brake horsepower that is developed statically under standard sea level conditions, within the engine operating limitations established under Part 33, and limited in use to periods of not over 5 minutes for TAKEOFF operation.

Looks like "cruise" ain't "takeoff" therefore...................


Fairly clear - if it makes TBO well that's another story altogether as previously posted.

rotorspeed

Interesting topic.

Question for Nick - assuming we're talking twins, to what extent are these beans in the engine jar or the transmission jar? What tends to be the ultimate limiting factor on power usage? Guess it varies according to acft and component design/evolution.

NickLappos

Crab said, "Nick's information would conclude that it is once only in the life of the engine and after it has been used the engine is irreparably damaged and is destined for a gradual decline in power output." Actually, your guidance to pilots is right on, but perhaps the concept of "Harm" versus "Use" needs clarification.

The idea that the helicopter is an immutible collection of objects, likely to do what it does forever, is not generally correct. Every operation "Harms" it a bit, some operations hardly at all, others with more rapidity. When you run your car, you "Harm" your fuel level, but you can periodically repair and renew it. Each cut with a saw blade dulls the blade, at some point it is not fit for service.

At 1000 hours of life in a 1000 hour part, the harm has reached a level that tells us it is wise to change it, but the part has adequate safety at 999 hours to fly the next hour.

But hours do not harm the part, the stresses on it do. More stress, less life. The stress is driven by the classic things, load factor, speed (rotor stall), gross weight, extreme CG, high altitudes, high power. We at the factory guess the way the aircraft will be flown (we call it the fatigue spectrum) by saying that each hour, the aircraft will fly x% at cruise, y% in climb, z% in hover, and that it will bank at 30 degrees p times, at 45 degrees Q times and so on. This list of educated assumptions is massed, and the test engineers then calculate the accumulated fatigue damage on the parts (fatigue damage is the "harm" Crab mentions).

When you operate, you do what you must, and the banks, speeds, altitudes and weights all sum to pull beans from the jar.

Transcients are established to cover the odd corners, where a slight increased torque or power might be seen, but where the manufacturer has confidence that a few of these in the life of the aircraft won't be enough harm to matter. Generally, we use transient power only as a cover for the oops! and not for actual usable power. We test the transients to show that a bunch of them during the overhaul life are not so additionally harmful as to cause concern. Use them as inadvertant exceedences, not as allowable surges, I think.


rotorspeed, all systems on the helo have some life inferred, transmissions, certainly. The TBO is calculated by figuring out when the beans will disappear, based on the assumed usage spectrum.

Generally, the longer the life, the stronger the part or system, of course. If the aircraft has lots of time change components, at low life, it is somewhat more marginal than one with few, long lifed components.

The ultimate limitation is usually the static strength of the part, which is the place where one pull at that level produces a break or permanent bending. Generally, a part can be stressed at 50% of its static strength for a great deal of cycles without any permanent harm (we call this the "Endurance Limit" strength.) For Sikorsky helicopters, we try to be sure that all components are below this endurance level for all level flight conditions, but maneuvers might pop up into the fatigue area.

Spaced

While I dont have any experience on twins, Im sure much of this debate can also be applied to singles.
Im still reasonably new to flying so I hope that this post doesnt expose my ignorance to much.
I have been following the posts so far with interest, however I note that most of the discussion has been related to the turbines. However arent most power limits, with a de-rated engine related to transmition limits?
I had been told that you can pull the 5 min limit, lower then pull again, however that didnt sound right to me.
Assuming that you are in a situation where high power is required, transiting in and out of the 5 min limit, due to weight, DA, whatever. Assuming that you are also in a slow speed environment, photo orbits, low level work, ect.... Meaning that your not actually in a "cruise".
Given this kind of scenario is there a correct proceedure for managing high power requirements. Are you just chewing extra time off the aircraft?
While Im not expecting an answer to cover every occasion, a little guidance here would be appreciated.

Gomer Pylot

Not necessarily. In many twins, you can reach engine limits well before transmission limits. It depends on the make and model, on the altitude and temperature, and on other things. On the model I fly, on a cool day the first limit I reach is Vne, but as it get hot I may very well reach N1 limits first, at least max continuous limits. I may reach transmission limits on takeoff, but not likely in cruise.

The 5 minute limit is for takeoff, and maybe to stop your too-hot approach, but not for photo work or anything else. If you're flying so slowly doing photo or any other work that you need to pull into the 5 minute limits, you're very wrong, and endangering both the idiot photographer who is asking for this, and the idiot pilot who agrees to do it.

If one engine quits at a bad time (and don't they always quit then?) then the other one had better watch out, because I'll pull all the power I need from it to keep flying - temp, N1, and everything else be damned. But that's not even close to the same thing as pulling all the power it can produce just because I want to go a little faster. My company bills by flight time, and the faster I go, the less money it makes, thus the less it wants to pay me. The driver referenced in the original post is plainly a pure idiot.

diethelm

I am certainly not the sharpest tool in the shed, and I am certainly no genius when it comes to operating a calculator let alone math and complex physics, but I was always under the impression that the more you abuse and beat the crap out of something the shorter it will last? I am also under the impression that this applies to most things in life whether they be mechanical or people.

The concept that anyone actually thinks that there is no correlation between continuously running a turbine in this manner and shorter TBO's would generally concern me about his/her general ability to have a clue.

heedm

We have a continuous mast torque limit of 81%, a 5 min take off limit of 100%, and a 5 minute limit of 100% when IAS < 105 kts. I acknowledge, as do most of our operators, that more cycles we have at the extremes of the envelope will accelerate fatigue, but I'm interested in the wording of the regulations.

The FAR that was posted was specifically about take off power, but by adding the identical limit for use in slower forward flight, does that section of the FAR still apply? If so, we wouldn't be breaking any regulations by reducing power for a short period to 'reset the clock' even in a take off condition (the second time above 81% would be the slower forward flight limit).

Has anyone seen any regulation that pertains to a 5 minute cruise limit?

NickLappos

heedm,
That limit is probably to help control a high mast stress that occurs when both torque and shaft bending couple at reduced speeds and high torque.

The 5 minute limit is intended to be per flight, to keep in the spirit of the fatigue spectrum, I believe. Has Bell given you any guidence?

IHL

heedm: May I ask what type of aircraft your flying?

heedm

It's a Bell 412EP that's customized to our requirements.

I haven't had a chance to check the flight manual, but I'm certain that routine hover operation is implicit in the specs since that much power is required to meet specified performance.

When above 40KIAS we don't use this limit. However it is required for almost all of our hovering work. I never liked the argument that 1 sec below 81% resets your 5 minute clock. I go into forward flight, giving about 4-6 minutes at ~60% between periods of hovering that require the higher setting.

Busy at work this week, but if I find anything more on this I'll post it.

Matthew.

detgnome

I believe that the limits for the 412 are thus:

You can't use more than 81% above 105kts due to mast bending considerations

When you are below 105kts you can use up to 100%, but are then constrained by the 5 min eng limitation.

Have to say we regularly use up to 100% Tq when engaged in fire fighting, but then again we are also at MAUW with the bucket attached - intrestingly with ambient conditions of approx +30deg C the ITT is topping on twin limits at 100% Tq suggesting a well matched power train. I still wonder how much extra that 19% would give in the cruise though...