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What Happens When You Redline Jet Engines?  
User currently offlineC5LOAD From United States of America, joined Sep 2008, 917 posts, RR: 0
Posted (4 years 6 months 2 days 4 hours ago) and read 7199 times:

How much more (in terms of percentage) thrust do you get during emergency situations when you have to redline engines? When you land, are those engines still usable, or do they have to be replaced?


"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
40 replies: All unread, showing first 25:
 
User currently offlineFr8Mech From United States of America, joined Sep 2005, 5327 posts, RR: 14
Reply 1, posted (4 years 6 months 2 days 4 hours ago) and read 7203 times:

IWhat are you redlineing? EGT? EPR? N1? N2? N3?

When you reach any single redline, you don't have to do anything. You've reached the safe limit, that's all. It is quite improbable that you will reach redline on all the parameters at any one time.

Now, you can exceed some limits, depending on engine, situation, and malfunction. The maintenance manual will dictate what checks need to be performed or, in some cases, whether the enigne needs to be pulled in some specified time frame or before further flight.



When seconds count...the police are minutes away. Never leave your cave without your club.
User currently offlineC5LOAD From United States of America, joined Sep 2008, 917 posts, RR: 0
Reply 2, posted (4 years 6 months 2 days 4 hours ago) and read 7194 times:



Quoting Fr8Mech (Reply 1):
IWhat are you redlineing? EGT? EPR? N1? N2? N3?

Sorry, I guess what I meant was something that my dad has told me was called "bending the throttles." Wouldn't this happen especially after a bird strike on a twin engine airplane on climbout when it's heavy? How many things redline when you bend the throttles?



"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
User currently offlineEMBQA From United States of America, joined Oct 2003, 9364 posts, RR: 11
Reply 3, posted (4 years 6 months 2 days 4 hours ago) and read 7191 times:

You do what the Engine Maintenance Manual tells you to do. Ever case is different, every engine is different. In most cases you boroscope the engine looking for heat stress and rubbing. You pull the DFDR and see what the engine was actually exposed too


"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineAlias1024 From United States of America, joined Oct 2004, 2741 posts, RR: 2
Reply 4, posted (4 years 6 months 2 days ago) and read 7082 times:



Quoting C5LOAD (Reply 2):
Wouldn't this happen especially after a bird strike on a twin engine airplane on climbout when it's heavy?

At my airline there are only two maneuvers where we push the thrust levers all the way to the stop, stall recovery and windshear escape.



It is a mistake to think you can solve any major problems with just potatoes.
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (4 years 6 months 2 days ago) and read 7062 times:



Quoting C5LOAD (Thread starter):
How much more (in terms of percentage) thrust do you get during emergency situations when you have to redline engines?

Zero, on most modern engines. Most FADEC engine will go to max certified thrust and stick there. There are a couple of weird cases; a CFM56-7 will go to the maximum certified thrust for the model, regardless of the programming plug in the EEC. Airbus has something similar, although I don't think it's implemented the same way. I believe that P&W used to offer a temporary overboost for certain takeoff situations.

But, for the majority of modern engines, pushing the throttle full forward will just give you maximum certified thrust which, by definition, can't be a redline.

Quoting C5LOAD (Thread starter):
When you land, are those engines still usable

Generally, yes.

Quoting C5LOAD (Reply 2):
Sorry, I guess what I meant was something that my dad has told me was called "bending the throttles."

This may be a term from the days before FADEC, when pushing the throttle beyond the normal max thrust actually could push more fuel into the engine (at the cost of engine life).

Quoting C5LOAD (Reply 2):
Wouldn't this happen especially after a bird strike on a twin engine airplane on climbout when it's heavy?

Remaining engine goes to takeoff thrust; this should never result in a redline on a modern engine.

Quoting C5LOAD (Reply 2):
How many things redline when you bend the throttles?

Nowadays, none. The engine should protect itself. Which component sets the limit usually depends on what altitude and speed you're at.

Tom.


User currently offlineMarkC From United States of America, joined Apr 2006, 259 posts, RR: 0
Reply 6, posted (4 years 6 months 1 day 18 hours ago) and read 6976 times:

As was said before, a rarity these days. The only time you usually see engine removals today due to too much power setting is with a cargo 727 with JT8's, and its usually because of windshear, and the critical parameter exceeded is EGT. Pilots do not seem to want to fool around with windshear. The engine manual has a graph. A certain # of seconds exceeded at high temperature. A certain number (more) exceeded at a lower temp, and so on.

Mark


User currently offline747classic From Netherlands, joined Aug 2009, 2076 posts, RR: 14
Reply 7, posted (4 years 6 months 1 day 18 hours ago) and read 6957 times:

There are still a lot of analoque designed aircraft remaining with non-fadec engines.
All 747 series, before the -400, L1011, DC10, all remaining DC8 series, DC9 series, Early A300/310's.

Aircraft powered by PW JT3, JT8 (except final versions), JT9 series, GE CF6-6,-50 and some early -80 series, RR RB 211 series until and incl.serie -D. All early CFM 56 series as on DC8-seveny series and KC-135.

If a coventional (non fadec) fuel control with mechanically connected throttles is present, in most cases overboosting the engine is possible by pushing the throttles to the end stop.
How much you can overboost depends on the outside air temperature, air pressure and regulator setting in the fuel control, combined with the rigging of the engine control cables system.



Operating a twin over the ocean, you're always one engine failure from a total emergency.
User currently offlineSaab2000 From Switzerland, joined Jun 2001, 1610 posts, RR: 11
Reply 8, posted (4 years 6 months 1 day 17 hours ago) and read 6931 times:



Quoting 747classic (Reply 7):
If a coventional (non fadec) fuel control with mechanically connected throttles is present, in most cases overboosting the engine is possible by pushing the throttles to the end stop.
How much you can overboost depends on the outside air temperature, air pressure and regulator setting in the fuel control, combined with the rigging of the engine control cables system.

This is correct. I fly the CRJ-200 which has no FADEC and no real overspeed/overtemp protections. It would be very easy to push the throttles past the recommended limits as there is no physical detent or stop in the throttle movement/travel. Just a marking on the engine gauges where to stop for T/O for CLB or FLEX thrust.

We once had a failure of the speed control and had to shut off the speed control in flight. The checklist calls for the engine in question to be reduced to near idle thrust before turning off the speed control. This was not properly done for a number of reasons not worth getting into here and upon landing the engine did have to be inspected for stresses induced, as it did go 'red' on several parameters.

FADEC is a good thing. I wish we had it.



smrtrthnu
User currently offlineBMI727 From United States of America, joined Feb 2009, 15695 posts, RR: 26
Reply 9, posted (4 years 6 months 1 day 12 hours ago) and read 6802 times:



Quoting C5LOAD (Thread starter):
When you land, are those engines still usable, or do they have to be replaced?

There was a story about a MiG-25 being clocked at Mach 3.2 and supposedly it ruined the engines. I would guess that some of the problems would have to do with parts melting inside the engine.

But luckily few pilots ever find out. Take care of your engines, and they will take care of you.



Why do Aerospace Engineering students have to turn things in on time?
User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 10, posted (4 years 6 months 1 day 10 hours ago) and read 6738 times:
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Quoting MarkC (Reply 6):
Pilots do not seem to want to fool around with windshear.

I remember hearing of an exchange between a pilot who had escaped some rather severe windshear on final and his employer. Apparently, the engines were overtemped, and damage resulted. Upon being asked why in the world he ran the engines up to 110% N1 (or whatever the value was), he replied "Because they wouldn't go up to 115!"



Intentionally Left Blank
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3971 posts, RR: 34
Reply 11, posted (4 years 6 months 1 day 10 hours ago) and read 6724 times:



Quoting Saab2000 (Reply 8):
This is correct. I fly the CRJ-200 which has no FADEC and no real overspeed/overtemp protections

I know nothing about your engine, but this sounds a bit backward to me. I worked on old engines. The RR Spey from 1965 had a fuel limiting system that reduced the fuel flow if the EGT limit was exceeded. It was no where near efficient as a modern FADEC, but it saved the engine.
The original RB211-22 from 1972 had a similar fuel limiter. It also controlled on N3(if I remember correctly) as well as EGT. Again, it didn't look at all parameters, but meant that if the crew firewalled the throttles the engine continued to function. The VIGV controller also mechanically limited engine speed. If the N2 went over limits, it closed the VIGVs to limit airflow through the engine.


User currently offlineSaab2000 From Switzerland, joined Jun 2001, 1610 posts, RR: 11
Reply 12, posted (4 years 6 months 1 day 8 hours ago) and read 6669 times:

Tristarsteve,

I should clarify what I am saying. There are actually overspeed protections, but no hard or fast stop for the throttle levers until it is simply all the way to the front of the movement, at which point you are probably overspeeding or overtemping the engine, or both.

But at 79.1% N2 it switches to an electro-mechanical speed control based on N1 and it does have some protections.

But no detents or stops like many of today's engines have.



smrtrthnu
User currently offlineStratosphere From United States of America, joined Sep 2007, 1651 posts, RR: 4
Reply 13, posted (4 years 6 months 1 day 8 hours ago) and read 6645 times:

I remember several years ago FedEx had 2 windshear events in the same week one in GJT and one in RNO both 727's one wound up being a 3 engine change and the other one was a two engine change. In most cases the FDR is pulled and boroscope inspections done to determine what action is taken.


NWA THE TRUE EVIL EMPIRE
User currently offlineAlias1024 From United States of America, joined Oct 2004, 2741 posts, RR: 2
Reply 14, posted (4 years 6 months 1 day 7 hours ago) and read 6609 times:



Quoting Saab2000 (Reply 8):
FADEC is a good thing. I wish we had it.

The one nice thing about not having FADEC on the CRJ-200 is that you can get a couple percent more N1 if you desperately need it. I know someone that had a windshear event on very short final at ABQ. With the thrust levers at the stop they were still descending toward the ground. He reached down and turned off the engine speed switches and they escaped, but just barely.

Review of the FDR showed an RA of 50 feet before they could get climbing again. Makes you wonder if they'd have hit the ground with FADEC.



It is a mistake to think you can solve any major problems with just potatoes.
User currently offlineDocLightning From United States of America, joined Nov 2005, 19275 posts, RR: 58
Reply 15, posted (4 years 6 months 1 day 3 hours ago) and read 6506 times:



Quoting 2H4 (Reply 10):

I remember hearing of an exchange between a pilot who had escaped some rather severe windshear on final and his employer. Apparently, the engines were overtemped, and damage resulted. Upon being asked why in the world he ran the engines up to 110% N1 (or whatever the value was), he replied "Because they wouldn't go up to 115!"

I think that those who are type-certified to fly desks sometimes forget that in an emergency, a maneuver that may shorten engine life may actually *extend* engine life by preventing the aircraft powered by those same engines from crashing into the ground. I'm no pilot or mechanic, but it strikes me as logical that crashes shorten engine life a lot more than 110% N1 does.


User currently offlineDw747400 From United States of America, joined Aug 2001, 1257 posts, RR: 1
Reply 16, posted (4 years 6 months 1 day ago) and read 6441 times:



Quoting DocLightning (Reply 15):
a maneuver that may shorten engine life may actually *extend* engine life by preventing the aircraft powered by those same engines from crashing into the ground.

Not to mention extending the life of the airframe, passengers, and crew.

I've heard of some FADEC aircraft (biz-jets) have the ability to boost power beyond the normal redlines in abnormal situations, like an engine failure or windshear. All of this is automatic--the crew puts the throttle to the stops and the aircraft systems as a whole determine if emergency power is needed (for example, airspeed drops sharply or one engine is outside of normal parameters). I can't remember which jet(s) this was on.



CFI--Certfied Freakin Idiot
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 17, posted (4 years 6 months 23 hours ago) and read 6424 times:



Quoting Dw747400 (Reply 16):
I've heard of some FADEC aircraft (biz-jets) have the ability to boost power beyond the normal redlines in abnormal situations, like an engine failure or windshear.

This is bending the definition of redline a little bit; redlining is taking an engine parameter (usually spool speed or EGT) past its limit. This is not the same thing as going beyond certified thrust. Due to the habit of engine makers to use one core (or one entire engine) for multiple thrust ratings, it's pretty common for an engine to be capable of more thrust than it's certified for. In fact, due to the way certified thrust is demonstrated, virtually all engines are capable of exceeding certified thrust without redlining.

For example, a CFM56-7B20 (~19,000 lbs thrust) can be run up to 27,000 lbs of thrust without busting any redlines, because it's physically exactly the same engine as a CFM56-7B27, just with a different programming plug in the EEC.

Tom.


User currently offlineDocLightning From United States of America, joined Nov 2005, 19275 posts, RR: 58
Reply 18, posted (4 years 6 months 21 hours ago) and read 6392 times:



Quoting Dw747400 (Reply 16):

Not to mention extending the life of the airframe, passengers, and crew.

Now you're just being a bean counter.  Wink

Quoting Tdscanuck (Reply 17):


For example, a CFM56-7B20 (~19,000 lbs thrust) can be run up to 27,000 lbs of thrust without busting any redlines, because it's physically exactly the same engine as a CFM56-7B27, just with a different programming plug in the EEC.

Why would you ever want such a bad bottleneck in an engine capable of so much more? Wouldn't it make more sense for an airline to ask for the full-power engine to be fitted to a given model, if it's going to weigh the same?


User currently onlineStarlionblue From Greenland, joined Feb 2004, 16975 posts, RR: 67
Reply 19, posted (4 years 6 months 21 hours ago) and read 6389 times:



Quoting DocLightning (Reply 15):
I'm no pilot or mechanic, but it strikes me as logical that crashes shorten engine life a lot more than 110% N1 does.

Brilliant statement. Darned right frameable.

Quoting DocLightning (Reply 18):
Why would you ever want such a bad bottleneck in an engine capable of so much more? Wouldn't it make more sense for an airline to ask for the full-power engine to be fitted to a given model, if it's going to weigh the same?

As I understand it the aircraft in question has weights that require only 19k thrust. The wear and tear on that particular engine, with a "max" of 19k, will be much less than on an equivalent engine set at 27k. So the airline has it restricted to 19k and can use the engine longer.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineRwessel From United States of America, joined Jan 2007, 2293 posts, RR: 2
Reply 20, posted (4 years 6 months 20 hours ago) and read 6368 times:
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Quoting DocLightning (Reply 18):
Why would you ever want such a bad bottleneck in an engine capable of so much more? Wouldn't it make more sense for an airline to ask for the full-power engine to be fitted to a given model, if it's going to weigh the same?

As Starlionblue mentioned, the maintenance costs on the lower rated engine may well be less.

A second (and in some case more important) reason is a bit of market segmentation by the engine vendor. Given the very high cost to develop and certify an engine, building *one* engine and offering it in several "models" allows considerably lower development expenses, while still allowing pricing to suit the application. And this basically benefits everyone, if done correctly (or just the vendor, if they’re greedy).

Consider the following hypothetical market for two engines, one 20,000lbs thrust, the other 25,000lbs. Let's say it would cost $500M to design, build and certify a "real" 20,000lb engine, with a unit production cost of $1M, and $625M to design the 25,000lb engine, with unit production costs of $1.25M.

Let's further assume that each engine will sell 1000 units over its design life. Amortizing the R&D expenses gets us $1.5M and $1.875M for costs for the two engines respectively. And then assuming a 50% margin on top of that leads to selling prices of $2.25M and $2.81M, which gives us total sales of $5.06B and a profit of $1.69B for the two programs.

Alternatively, the manufacture could just build the bigger engine, maybe toss in an extra $100M R&D to make it a bit better (a bit little lighter, a bit more fuel efficient, etc.), a bit cheaper to manufacture (let's say they can save $100K/unit), and then just make a different program ROM for the FADEC.

Now the manufacturing cost for each for the *2000* engines is $1.15M, and the amortized R&D ($725M) gets us a total cost of $1.513M/unit. Now the manufacturer plays with the margins a bit, and let's say sells the "small" engine for $2.15M and the large one for $2.7M (getting a slightly lower margin on the small engine and a slightly bigger one on the big engine), which yields total sales for the program of $4.85B, but total profits of $1.82B (an absolute higher profit, *and* a higher profit margin, with lower total costs).

The customer, OTOH, gets cheaper engines, that are better (since more R&D was spent). It's an absolute win for the big engine customer on all fronts, and the small engine customer gets higher reliability, lower costs, etc., in return for a small weight penalty. Everybody gets cheaper and more plentiful spare parts. In short, everybody does better.

The obvious question is why not sell all the engines at one price? Because that doesn't work - it's great for the big engine customer (who could now buy their engine at $2.43M), but you lose all your small engine sales because the engine is overpriced for its market. So basically the segmentation exists to distribute the costs (in this case for the higher thrust) to the customers who need that feature. But it does lead to the (apparent) unfairness of the manufacturer charging two different prices for the same bit of hardware.

It can even help the airframe manufacturer beyond the obvious advantage of having cheaper engines – this can make it much easier to offer a “hot-and-high” (or similar) package for your aircraft. Which, for example, is exactly what Boeing does with the 777-200LR, where, for a few extra dollars, you can get an extra 5000lbs thrust per side by swapping the plug to turn the GE90-110s in to -115s. And only the customers who need the bigger engine pay for it.

Note that the computer industry is well known for that particular tactic. Back in the late seventies there was a well known bit of documentation floating around the mainframe world about how to rewire a jumper block in a certain third-party-vendor's tape drive, to transforming the tape drive from the slow to the fast model, or to enable support for several (optional) tape densities. Needless to say the vendor was very unhappy about this, and you had to swap the “official” jumpers back in before placing a service call.

There have been many examples since then. A very common one being the microprocessors themselves – they’re often offered in a range of speeds, even for the same physical chip design. And while some amount of that range comes from testing – IOW some chips test faster than others due to small manufacturing variances – much of the binning is artificial, especially as the production line matures and most of the dies start testing out at, or near, the “maximum” speed. All the folks who like to “overclock” their CPUs depend on that.

And as the ratio of R&D to (program) manufacturing costs increases, the benefit to this sort of scheme increases. Of course the reverse is true as well - as program production costs increase relative to R&D costs, it becomes increasingly better to develop dedicated models of product optimized to each required feature set. Of course a complicating factor is that building two products may not take two full R&D efforts, rather one might be a derivative of the other - that doesn't change the principal, it just moves the crossover points around.


User currently offlinePellegrine From United States of America, joined Mar 2007, 2324 posts, RR: 8
Reply 21, posted (4 years 6 months 18 hours ago) and read 6337 times:



Quoting BMI727 (Reply 9):

There was a story about a MiG-25 being clocked at Mach 3.2 and supposedly it ruined the engines. I would guess that some of the problems would have to do with parts melting inside the engine.

But luckily few pilots ever find out. Take care of your engines, and they will take care of you.

I would defer to the metallurgists and engineers. But I am sure in this case and others of excess EGT, the limiting factor is the HP turbine and possibly IP/LP turbine. The high temperature metals (nickel or stainless steel most probably) will "creep" far before they melt. Meaning they won't just discombobulate and fly out the back end, but will stretch at the high RPMs a turbine operates at and become un-useful and potentially dangerous to engine operation by throwing the turbine disk out of shape.

Quoting Rwessel (Reply 20):
All the folks who like to “overclock” their CPUs depend on that.

Been overclocking since the 80386.  Wink



oh boy!!!
User currently offlineFaro From Egypt, joined Aug 2007, 1533 posts, RR: 0
Reply 22, posted (4 years 6 months 17 hours ago) and read 6316 times:



Quoting Tdscanuck (Reply 17):
For example, a CFM56-7B20 (~19,000 lbs thrust) can be run up to 27,000 lbs of thrust without busting any redlines, because it's physically exactly the same engine as a CFM56-7B27, just with a different programming plug in the EEC.

Using the full 27'000 lbs of thrust is a no-no if you are in an engine-out situation so the programming plug applies in this instance. Why not cater, however, for situations where that extra 8'000lbs of thrust is suddenly and direly needed? Surely that programming plug can be smart enough to discern if there is an engine out or not...

Faro



The chalice not my son
User currently onlineStarlionblue From Greenland, joined Feb 2004, 16975 posts, RR: 67
Reply 23, posted (4 years 6 months 16 hours ago) and read 6307 times:

Well, the 19k are supposed to be enough for any emergency. Sure, you always want "more power" but note that the aircraft is certified to handle "any" emergency with 19k per engine. Also, wouldn't an extra 42% power result in some unplanned yaw effects with an engine out?


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFaro From Egypt, joined Aug 2007, 1533 posts, RR: 0
Reply 24, posted (4 years 6 months 15 hours ago) and read 6291 times:



Quoting Starlionblue (Reply 23):
Also, wouldn't an extra 42% power result in some unplanned yaw effects with an engine out?

Yes indeed  Smile:

Quoting Faro (Reply 22):
Using the full 27'000 lbs of thrust is a no-no if you are in an engine-out situation so the programming plug applies in this instance.




The chalice not my son
25 Fabo : Doesnt that result in mandatory de-rating of one-engine-out on light 777 take-off or something like that?
26 Jetlagged : It was called TTC (Top Temperature Control). Only a limited authority though. The Spey also had an N2 governor, N1 limiter and a P3 limiter. Rolls Ro
27 Tdscanuck : It depends on Vmcg/Vmca for your particular airplane. I don't think a 737-600 is certified for the -7B27 (too much thrust), but it can certainly hand
28 Post contains images BEG2IAH : This is how it looks. I was amazed how small the plug is, but does so much for thrust limitation. This was shot at OSL in SAS hangar. BEG2IAH
29 ThirtyEcho : I am very familiar with this term. I have quite a bit of piston time, going back to 1959, and my uncle was a B-24 command pilot during WWII. It simpl
30 Brons2 : In 1944, maybe a Fw-190 was closing on your tail? Although in a B-24, I don't think any amount of throttle bending could make the plane outrun any of
31 Tdscanuck : Just so nobody gets weird ideas about my father, that's actually C5LOAD's quote from Reply 2, not me. Tom.
32 C5LOAD : Thanks, so you think my father's weird do you?
33 JCS17 : Actually, one of the criticisms leveled after the Air Florida crash at DCA was that the throttles were never firewalled, and remained at a reduced th
34 Tdscanuck : Nope. Just that any suggestion that *my* father flew an airplane or knew what "bending the thottles" meant would be very weird. Tom.
35 Flighty : It seems that jet engines have an exponential reduction in service life as they increase thrust. For example, reducing the thrust % by a certain perce
36 Starlionblue : Yes it is. It is worth noting, however, that it is less costly than a crashed aircraft.
37 747classic : We all agree that passing the red line (operating limits) of an engine can be very expensive. But there are some situations that you want to have all
38 Tdscanuck : There are limits to how far you can extend this logic...at some point, the engine becomes a danger to you (rotor burst), possibly a larger danger tha
39 747classic : Tom, I have to disagree with you on this subject. I know the blessings of a FADEC or ECU controlled engine. In 99,9% of all engine failures it will he
40 Tdscanuck : But the computer is trying to do the same thing you are...maximum thrust without having the engine come apart. I hope you'd agree that having the eng
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