Jet Engine Overboosting

[bflyer]

Hi gentelmen

Your inputs on the following is greatly appreciated

1) A definition of the term OVERBOOSTING

2) The conditions which will lead/may lead to overboosting a high-bypass
turbofan engine ( e.g neglecting to apply proper bleed correction to epr
values at a certain phase)

3) Indications from engine instruments ( epr,n1,n2..etc) that an engine is
beign overboosted

thanks for your time

[IFixPlanes]

Where did you read the term "OVERBOOSTING" in conjunction with "turbofan engine"?

[A2QFI]

I have only ever heard the term overboosting applied to piston engines and that was in the context of a non super or turbo charged Lycoming fitted to a Bulldog.

[Intruder]

It's been discussed recently, but I can't find the thread.

Essentially, it's a term used by some instructors to describe a condition where the engine is producing more than its design/certified thrust.

For example, our 744 isstructor used the term to tell us the EECs on the CF6 engines protected against overspeed and overboost (overthrust?) in the normal mode, but overspeed only in the alternate mode.

[Rainboe]

On some engines, not all engine parameters are limitation protected. On our RB211s, we had no protection on EGT. Other parameters were protected from exceedence. On one very heavy and hot take-off from Bombay, we had a power shortfal on a couple of engines. We had an allowed shortfall of power limit- .02EPR per engine or .06 across all 4 engines. It took some pretty rapid calculations by the Flight Enfgineer who could call Go/No Go. I drew attention to it, and being new on type, he pushed the thrust lever open to get rid of it. We had several red EGT lights on during the take-off. When airborne we looked at it and realised we'd exceeded max EGT by trying to chase N1/N2/N3 and EPR up to limits. It resulted in an engine change, 250 pax in hotels in Perth, and a special ferry flight to bring in a new engine (and a Qantas mechanic sliding off the top of the engine in a storm and breaking something).

That's not going into my memoirs!

[BOAC]

I'm sure it is a 'hangover' for piston as said, but Boeing use it in the 737 FCTM.

[lomapaseo]

I guess that you will have to search the various engine manuals to see if they use the term overboost
In engineering venacular it is used to apply to a condition where any of the critical parameters are exceeded in the gas generator. the idea of the engine control system is to protect against these exceedances to an extent.
In the rotors it's an overspeed that is the concern. Too much RPM can get you into big trouble with the vibratory integrity of critical rotors (Ala the National airlines, DC10/CF6 that spewed out the fan blades)
Too much overtemp and you could burn out the turbine (pretty common)
Too much overpressure is unlikely to be a problem on its own (the engine cases have margin) but it typically comes with baggage like overtemperature as well

[barit1]

Agreed the term "overboost" is carryover from recip engines, but is nonetheless used on turbines.

EGT (or ITT) protection is generally not provided for big fan engines, on the theory that an alert crew can bring an engine back within limits without exceeding the time-temperature envelope.

Rotor speed protection generally has double or triple redundancy for dangerous overspeed, but any time you exceed the published power management value (N1 or EPR) for existing ambient conditions, some kind of maintenance check is called for.

Since you probably don't have CDP (burner pressure) readout on the panel, you can't do anything about it. But rest assured there is overpressure protection, since a burner pressure vessel burst is about the last thing you'd like to happen!

[rubik101]

If you push the throttles/power levers fully forward will you damage the engine?
You might produce more power/epr than the rated thrust but I doubt you will damage it.

[411A]

So far as I know, auto TGT limitation was only provided for on one large RR turbofan...the RB.211-22B.
Some operators disconnected the TGT channel from the fuel control amplifier, to prevent undue difficulty.
On takeoff, the fuel control amplifier would roll back thrust if temp limits were approached...sometimes thrust was rolled way back, and on a limiting runway, could catch crews by surprise.
However, if the alert F/E noticed this straight away, the Captain could quickly retard the affected throttle to 1.2 EPR, the F/E would unlatch the FCU to 'override', and thrust would then be re-applied.....weight and runway remaining, of course.
This takes good crew co-ordination to work effectively.

[Mad (Flt) Scientist]

If you push the throttles/power levers fully forward will you damage the engine?
You might produce more power/epr than the rated thrust but I doubt you will damage it.

At a minimum you'll be consuming the economic life of the engine faster than the normal rate, which is damage of a kind.

Go sufficiently high in terms of power and yes, you will damage stuff pretty quickly.

[VnV2178B]

The Trent series of Rollers do not detect a TGT exceedance directly but do flag a possible exceedance using the shaft speeds. A maintenance message is also generated to warn of the overthreat and make sure the innards are checked before continuing.

If the speeds get really silly then the engine is shut down by the FADEC before too much damage happens (hopefully...)

VnV

[Rainboe]

As far as I remember from all types I've flown, it's guaranteed that you can firewall the throttles to get yourself out of trouble. But the engine will have to be removed.

[BelArgUSA]

A few years ago, my airline had leased an additional aircraft, a 747-238B to supplement our fleet of 747s...
xxx
The airplane was reported to have JT9D-7J engines (there was a little "7J" label on top of each row of engine instruments), and we often used maximum power for takeoff, as that aircraft was used as back-up for our 747 fleet which is normally powered by JT9D-7Q engines...
xxx
If I remember well, there was an engine (nº 3) constantly showing higher EGT when we went to maximum EPR equal with the other engines... None of the crews flew that airplane often, so we did not mind. Was probably for a year or even more time... Until one day...
xxx
Maintenance found out that engine nº 3 was actually a JT9D-7AH... So that engine got "overboosted" (if you use that vocabulary) for hundreds of takeoffs at -7J EPR settings... Eventually, we got a -7J on that position, but it demonstrates that some engines can take a lot of punishment of the "overboosting" nature.
xxx
Makes me smile, I know that a JT9D-7AH is actually a JT9D-3A that has been upgraded to JT9D-7A standards. Must have been a very "old" engine near retirement, yet it survived all these years, giving 50,000 lbs of thrust for takeoff, when only rated for 46,150 lbs thrust...
xxx
:)
Happy contrails

[rubik101]

Mad Scientist, I have to take issue with your assertion that operating an engine at max power will damage it. I accept that using reduced or derated power prolongs the life of the engine but to refer to full power as damaging power is not correct.

[Mad (Flt) Scientist]

Throttles fully forward (which was your phrase) can be WELL in excess of max power on a hydro-mechanically controlled engine; on a CF34 you'll get as much as 25% extra thrust if you firewall the engines.
As stated by rainboe, you CAN (and should!) push the throttles fully forward in a true emergency - but you'll have trashed the engines in the process.

[lomapaseo]

With today's FADECs you can push the throttles through the windscreen and you may not be able to overboost the engines and get any more thrust.

If you want the feeling of extra power to spare, then go with a lighter load.

[rubik101]

Mad, I beg to differ. So called 'Firewalling' modern jet engines will not 'trash' them nor even damage them. A jet engine cannot produce more power then it was designed to produce, that being its MAX Thrust. It can produce more power then it is rated to produce if the throttles are pushed fully forward. It is an urban myth that an engine will need to be changed after such action. If CFM/Boeing thought that such an action would 'trash' their engine then they would not refer to it in the QRH as being allowable in extreme conditions. Provided the engine is not kept at max power longer than the allowed time, normally 5 to 15 minutes, it will be perfectly usable in the future. It might be subjected to some examination but it will most certainly not be 'trashed'.

[barit1]

Well, it all depends.

The FADEC sophistication gives the designer the ability to make the engine highly "bulletproof" - he can readily make the software protect the engine from almost any overboost. (Albeit, as mentioned before, EGT/TGT/ITT limits are still left to the pilots to observe...)

Older engines - pre-FADEC - have to rely on that bucket of gears, cams, valves, and bellows called a FCU or MEC or whatever, and there's only so much hardware you can cram into that device. So, it probably doesn't give the degree of engine protection younger pilots have experienced.

:)

[Mad (Flt) Scientist]

@rubik

The CF34 is a hydro-mech engine.

The rated max takeoff power can be achieved with the throttles some distance from the stops, depending on conditions. Push them forward and you'll exceed the rated N1, and hence the rated thrust. By, as I said, as much as 25% in some conditions.

There are more engine/airframe combinations than CFM and Boeing: be wary about making sweeping statements about what is or isn't impossible extrapolating from a single design approach.

If you encounter Windshear in a CRJ (for example), you push the throttles as far as you can if you have to (the exact phrase is use "all available thrust" to avoid ground contact); so what if you trash the engines (and that's a definite possibility at 25% overthrust), at least you might save the aircraft....

[Denti]

Yup, we had the case recently on a 737-300, old style CFM56 (no FADEC) got overboosted during a windshear manoeuver around 20 feet during takeoff, both had to be replaced as per manufacturer instruction for that case. Funny thing is both pilots reportet they saw overheat, however the QAR only showed N2 overspeed.

[gas path]

The Jt9d was a 'part throttle' engine. It would give you what ever you required in the way of thrust, up until the 'speeder spring' in the JFC limited the RPM.
The RB211-22 up to the RB211-524d the fuel control amplifier limited thrust.(N1 and N2)
As 411A states some operators had the EGT channel whilst others, BA for one had it isolated. However if the amp was switched out the only limit came from the FFR, hence the requirement to throttle back before switching the amplifier to override.
The Trent series of Rollers do not detect a TGT exceedance
B777 the EGT EICAS figure turns red for an exceedance and returns to white as the overtemp comes back into limits. However the EGT 'box' remains red until the exceedance page is erased.The exceedance page gives a timeline of temperature and time above.

[wileydog3]

You can overtemp and engine and you can overspeed an engine and both require logbook entries on every jet engine I have throttled. But overboost is a recip term and unfortunately it persists among even many training facilities.
And where this becomes a problem is when some training has the student figure a maximum N1 or Max EPR or whatever before doing a stall series. Rubbish, to use the term. Unless one considers stalls a normal procedure then normal limits do not apply. Firewall or MAX thrust until the thing quits shaking and the stall is broken.
I thought this concept of max EPR on recovery had been invalidated in the 80s with Air Florida 90 going into the Potomac but some still teach stalls in businessjets this way. ?????

[ahramin]

A turbine jet engine always has by its very nature the ability to destroy itself. Whether or not this can be done from the flight deck by bending the power levers over the front depends on what controls the aircraft manufacturer has put in place to prevent this from happening.

Case in point our company had a turbine engine in Africa on a natural gas pipeline (used to "pump" the gas down the pipeline). The pipeline sprung a leak and the engine was sucking in some of this gas so it started to run faster, which pumped more gas out as the leak was just downstream. It just kept going until it (very quickly) self destructed due overspeed.

It doesn't matter what an engine is rated for, if you put more fuel in, it will produce more power, until it breaks.

[Zenj]

In FCTM for B737-200 Boeing has a section for Overboosting

This is what it says !

Engine Overboost 7.18

A significant thrust overboost capability exists which could be used in emergency situations. This overboost capability should only be considered when ground contact is imminent.
Overboosting the engines when the situation is not sufficiently serious unnecessarily shortens engine life and increases the potential for engine failure.
In an emergency situation “firewallingthe thrust levers” should be considered.
This condition could result in an EGT or N1 exceedance.
Land at the nearest suitable airport

[Jonty]

In our company Vol 1 it talks about overboost/overspeed protection. On the RB211 we dont have FADEC but we do have an EEC to stop overboost overspeed. So we can push the thrust levers fully forward and not trash the engine, although we have no EGT protection. Without EECs we have no overboost/overspeed protection and therefore we can trash the engine.

This relates to the RB211E4 on the B757

[exeng]

I remember your incident very well (November 1984). I was on secondment to QF from BA as an F/E and was called from standby to operate a 5th pod flight (SYD - PER) to provide your service with an engine.

We all met up in the Parmelia for a bit of post flight refreshment.

I remember discussing the incident at length with your F/E who was more than a little concerned as to what may be in store upon his return to LHR.


Regards
Exeng

[BelArgUSA]

Sadly, there was a perfectly competent French A320 test pilot that probably would have loved to overboost his engines after a low pass at an airshow, but ended in the trees... but all the electronic junk you have nowadays will not permit to save an airplane and passengers from an accident... If and when I need power, my 53,000 lbs thrust engines will give me 55,000... And I will bend the thrust levers if need be...
xxx
Keep your FADECs for yourselves, Gentlemen, none on my old 747 Classics...
And I love it that way... Ah... the good old days...
Soon at the beach in Brazil, and forget forever all about EGT or N1 limits...
Forever
xxx
:)
Happy contrails