747 Takeoff and cruising power comparison 1

[TPL]

Can anyone give me some pointers as to the power required by a 747 powered by RB211 engines for takeoff, compared to the power required for the same aircraft to maintain a typical cruising altitude/speed.

I know there are lots of varaibles but I am looking for ball park figures

thanks in advance

 

[IRstuff]

> Lifting a very heavy plane up into the air

> Overcoming drag at speed

TTFN

FAQ731-376

 

[TPL]

I was hoping for something along the lines of:

If the fully loaded aircraft requires 95% of available engine power for taking off, then at cruising height and speed the engines are throttled back to around X% of full power.

Any ideas of typical values of X?

 

[FieldTeam]

Simplified:
Full power take off is just that. The engines are brought to full power and when the aircraft reaches a certain speed (Vto), the aircraft leaves the ground.
The crew could also elect to perform a reduced power take off. The aircraft charts would be used to determine this power setting.

Some conditions that influence take off performance would be aircraft weight, temperature, pressure altitude, wind and thrust.

RB211-524B2, 50,100# thrust.
RB211-524D4, 53,000# thrust.
RB211-524G2 with 58,000# thrust.
RB211-524H with 60,600# thrust.

 

[swearingen]

I've never flown the real thing, but from the simulators I've flown, it takes as much or more power to cruise at altitude than to take off.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[TPL]

Thanks but I really am looking for something a lot more simple.

I work in oil and gas and look after a fleet of RB211 aeroderivative engine used in gas compression duties.

One of the company senior managers has been contacted by a 'business management consultant' who has stated with some 'justifications' that that aircraft RB211 engines are typically overhauled each 3000 take off cycles, with overhauls typically 7 years apart.

We typically overhaul our RB211s at 30k operating hours or about once each 4 years - so the snake oil salesmans, consultants figures look as if they can make a pretty good contribution to our bottom line at first glance.

The difference is that our RB211s operate all the time at or around full load and aircraft engines only pull full load at take-off.

So, an aircraft engine might only operate at full load for around 3000 hours in a 7 year period, whereas an aeroderivative engine in gas compression or power generation duty might operate close to full power for 25k hours in a 3 year period.

I have been asked why we don't overhaul our engines at 7 year intervals, since aircraft engines have to be more strictly maintained: my response is going to be based on the knowledge that the life of a GT is determined by hot end operating temperatures which are determined by load

So, in order to pacify management that we are doing the right thing I am assembling a small presentation describing the differences in operating regimes between aircraft engines and their industrial derivatives - as part of this presentation, I would like to know what percentage of full load power is required to maintain a 747 at a typical cruising height and speed.

I am not looking for precise data, just a information along the lines that "a typical 747 requires (say) 50% (or whatever a realistic value is) of take off power to maintain cruise height and speed"

 

[GregLocock]

Let's see, a Nimrod (basically a Comet) can cruise on one engine out of four running.

Concorde took off at 38000 lbf thrust per engine, cruised on 10000 lbf thrust (at M2+)

You can hear jets throttle back when they hit cruising altitude.







Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

your usage (100% full power) is much more severe than a typical airplane. Your maintenance time between overhauls is (clearly) significantly more than a typical airplane.

consider a couple of points ...
1) does your maintenance show up issues ? i'm not sure what parts of the engine are most sensitive to fatigue (though the hot section would be my guess).

2) what are the consequences (and costs) of failure ? for an airplane they are pretty severe so the MTBO is shortened. what about your operation ?

3) how long have you been maintenaing the engines this way ? if for a while it would suggest that you're doing nothing particularly wrong.

maybe RR might offer some (knowledgeable) insight ? i'd expect that they'd suggest shortening your maintenance cycle !

 

[TPL]

I don't know anything about aircraft, other than they take me from A to B.

Greg - Yes you're right, the flaps go back and the engine power drops at cruising speed

rb1957 - thanks for your insight but I already know all this.
1) No maintenance issues. The principal sources overhual cost are blade failure and combustor distress - burning away of protective coatings
2) Consequences of failure are loss of production i.e. financial. The biggest risk to safety is blade loss so, apart from performance and overhaul cost, there is a safety risk with extended running.

3)For ever - just like everyone else - but with experience, and using condition monitoring, oil analysis, performance and borescope inspection, we can extend overhaul intervals from the OEM recommended 25khr to 32khr. The longer we run between overhauls, the more of the hot end needs to be replaced and the greater the cost of overhaul. However, the cost of overhaul divided by the hours since last overhaul is fairly constant.

As I tried to explain earlier, a business consultant has turned up and asked our management why we don't have RB211 overhauls at 7 year intervals like the aviation industry - with a typical overhaul costing US$2 million, spinning the overhaul intervals out to 7 years is an attractive proposition, but its not an 'apples with apples' comparison, hence my request for approximate power consumption during cruising. I would like to be able to quantify the differences in power consumption

 

[IRstuff]

I think you've answered your own question:

The longer we run between overhauls, the more of the hot end needs to be replaced and the greater the cost of overhaul

Your "forever" statistics of existing overhauls should give you an idea of how often you need to do serious repair/replacement vs. interval between overhauls. It sounds on the face of it, you're at the edge of where you need to be do prevent in-use failures, which is the whole point of doing maintenance. Clearly, at double the interval, you should be able to show that there'll be X% of engines that will require $Y repair, and Z% of catastrophic failures, with, perhaps a ZZ probability of death or injury to personnel/equipment.

TTFN

FAQ731-376

 

[TPL]

IRStuff - I have not answered my own question. I simply want to know what proportion of available engine power is required to maintain cruising speed and altitude of a 747.

I am obviously framing this request for information very poorly.

There is nothing fundamentally wrong with the way we maintain operate and overhaul our RB211 engines each 4 years or so (based on operating hours and condition).

An alleged 'business consultant' has sown the seeds of doubt in senior management by saying that the aviation industry operate their engines in a far more severe environment but only need to overhaul their engines each 7 years. The implication here is that oil and gas operators should be able to achieve the same 7 year overhaul intervals: management have come back and said that if the aviation industry can overhaul their RB211s each 7 years then what are we doing wrong?

I could just say that a 747 on long haul duty operates most of its time in cruising mode. This means that its engines spend most time in low power regime, with consequent low working temperatures so that the hot end does not burn up as quickly as working at fullpower, but I would like to try and add some numbers, even crude ball park figures.

 

[rb1957]

"As I tried to explain earlier, a business consultant has turned up and asked our management why we don't have RB211 overhauls at 7 year intervals like the aviation industry - with a typical overhaul costing US$2 million, spinning the overhaul intervals out to 7 years is an attractive proposition, but its not an 'apples with apples' comparison,"

"1) No maintenance issues. The principal sources overhual cost are blade failure and combustor distress - burning away of protective coatings" ... doesn't quite jive with "The longer we run between overhauls, the more of the hot end needs to be replaced and the greater the cost of overhaul."

"2) Consequences of failure are loss of production i.e. financial." ... would that be huge (shuting down the plant for a day, a week?) or reasonably minor (momentary production fall whilst you switch in the reserve engine) ?
"The biggest risk to safety is blade loss so, apart from performance and overhaul cost, there is a safety risk with extended running." ... uh? if a HP disc broke and spread it's components (with inifinite energy) across your plant would something (or someone) get hit ?

"3)For ever" ... ok let's say you have lots of experience that what you're doing protects the engines nicely
"but with experience, and using condition monitoring, oil analysis, performance and borescope inspection, we can extend overhaul intervals from the OEM recommended 25khr to 32khr. The longer we run between overhauls, the more of the hot end needs to be replaced and the greater the cost of overhaul. However, the cost of overhaul divided by the hours since last overhaul is fairly constant." ... so you can extraploate the cost to opening this up to 7 years and add in the increased possibiliy of having a disc rupture.

i think you wish you could tell the "consultant" to eff off (and maybe your boss for listening to them??), that your operation doesn't compare to a jet (der) so your maintenance is difference, and you've already stretched to industrial maintenance recommended (25k to 32k). actually it sounds as though you have lots of good data ... the OEM is recommending one thing for industrial apps and another of planes, and i suspect that most planes are limited by 3000 hrs rather than 7 years.

 

[bf109g]

I agreee w/ rb - your operation does not involve flight, so why are you trying to compare to a 747 (or a 757 for that matter)? If an engine fails in flight, it's a much more critical situation than the same failure in an industrial facility (provided that the industrial facility has the proper shrapnel protection).

I'd stick to the RR recommendations for industrial apps and your "tribal knowledge."

 

[TPL]

This isn't going the way I hoped.

"I think you wish you could tell the "consultant" to eff off" - spot on, but his stance is seductive to the bean counters and I would like some numbers to support my position.

"1) No maintenance issues. The principal sources overhaul cost are blade failure and combustor distress - burning away of protective coatings" ... doesn't quite jive with "The longer we run between overhauls, the more of the hot end needs to be replaced and the greater the cost of overhaul."
For blade failure, I should have written blade rejection - at overhaul, a selection of blades is destructively tested for creep and life assessment - nearly all hot end components (blades, combustors) are coated with thermal barrier material - this does not have an infinite life and any breach of these coatings (hot gas erosion, fod, corrosion etc) means loss of base material and component rejection/replacement. The longer we run, the more components we have to replace - rather than remove and reapply the coating.

"so you can extrapolate the cost to opening this up to 7 years and add in the increased possibiliy of having a disc rupture" - the disc cycle life is monitored by the OEM (using our operating data) and we would not authorise reinstallation of a disc that did not comfortably exceed OEM life requirements.


"i suspect that most planes are limited by 3000 hrs rather than 7 years" - its effectively 3000 cycles which can take as long as 7 years to achieve.


"so why are you trying to compare to a 747 (or a 757 for that matter)?" - I wish I wasn't but some slimeball sales rep who probably wouldn't know a gas turbine if he was hit on the head with one has been taking this approach to drum up work.

 

[rb1957]

you poor b@st@rd, someone's latched onto the "7 years" like a pit-bull.

there really is no point to comparing your usage to an airplane's when you have OEM data supporting your usage.

you could ask RR what their calendar time limit is.

you can extraploate your costs so that a 7 years MTBO would correspond to >60k hrs. curve fit what you know, maybe linear, maybe parabolic (nudge, nudge ... you can adjust the future to suit your conclusion !). sounds like you've never had a disc rupture, and probably don't want one ... the longer you run between overhaul the greater the chance of something going amiss. you can probably paint a pretty nasty picture of this.

at the end of the day, you can only recommend a course of action, and someone else is making the decision; just make sure that that someone else can't pin the blame on you later when things go awry. (CYA?)

 

[bf109g]

Yep - sounds like you're hosed.

Working for XXX and flying one or two cycles a day, we were able to extend the calendar Mx tasks. What American would have to do every 18 months, we were able to "escalate" (never did like that term) to 30 or 36 months. And the FAA bought into it.

The green eyeshade types were so locked into the immediate cost savings, that they didn't notice that we were flying the things more than the two cycles per day due to volume increase and aircraft retirement. So we ended up with a Mx plan that would have been valid 2 years prior. Lo and behold, things were failing on the line long before they were due to be checked under the cheaper plan. Of course, all of those involved in the escalation were rewarded and moved on to bigger and better things. The rest of us slobs were left to mop up the mess.

Sounds like you are in a similar sit, so be highly protective of your posterior.

 

[IRstuff]

My interpretation is that your "question" is whether 3 yrs or 7 yrs is the correct maintenence period, isn't it? What the power level on a 747 is of academic interest, unless you know exactly what power level is during each and every minute of its usage life.

Your consultant is making an academic comparison, he has nothing more than some industry practice. YOU have actual data on YOUR equipment. The statistics of your operation is what's important. Use YOUR data to refute the arguments, if it supports your position. Whatever the consultant could say cannot supplant actual data from your own operations.

TTFN

FAQ731-376

 

[FieldTeam]

Starting, accelerating to and from power, EGT margin and cool down (low and high cycle fatigue) of aircraft engines are some of the factors you can't compare to your steady state situation.

Maybe MIL-HDBK-1783 can give you something as a reference for your case.

 

[btrueblood]

Just to add another $.02 - the "life" of an aero engine is considerably shortened by the number of stops and starts it undergoes, each adding a bit of wear to bearing, seals, rub strips, etc.

The answer to the accountant's arguement ("why can't we go 7 years?") is "we should be overhauling at 3,000 hours, not 30,000 hours". Damn spin doctors.

 

[swearingen]

The engine sitting on the shop floor isn't being bounced around on its foundation (turbulence, landings, & expansion joints while taxiing) and I would think the pressure and temperature differentials that an aircraft engine goes through would make a big difference as well.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS