fire-walling" CFM 56 thrust levers in emergency go-around.

[Tee Emm]

The Boeing 737 Classic QRH advises to aggressively apply maximum thrust following a GPWS pull-up warning. It amplifies max thrust by saying it can be obtained by advancing the thrust levers to the take-off or go-around limit. If terrain contact is imminent, advance thrust levers full forward.

In the simulator "firewalling" the thrust levers does not result in an over-temperature indication. It is known that instances have occurred when inadvertant firewalling of CFM 56 engines has resulted in an over-temp condition. Beyond a certain EGT limit, the engines may have to be inspected for damage.

The question arises thus: If the thrust levers are advanced to the stop during an emergency, is there a safety feature incorporated in the engine design to prevent over-temping beyond the normal EGT limit for take off or go-around thrust. If an engine does overtemp during a max thrust go-around, does this mean the safety feature has failed in its task to prevent a damaging over-heat temp? I seem to recall the RR Spey engine on the F28 had a top-temperature limiting device where fuel flow was automatically reduced if EGT limits were approaching exceedance.

Last year a go-around at max fire-wall thrust resulted in an over-temp which was recorded via the FDR. The pilot was sacked. The circumstances are unclear but would it be correct to say that the over-temp safety feature (if fitted on a CFM 56) should have theoretically cut in to prevent an overtemp sufficiently serious enough to require engine change?

There is no mention in Boeing publications warning of this overtemp danger if the thrust levers are firewalled in an emergency. If in fact overtemping is likely to occur during a go-around, one would think this should be incorporated in simulator design? As it is now, firewalling the thrust levers if ground contact is imminent is a recommended Boeing procedure and thus practiced in the simulator with no apparent adverse indications.

[de facto]

As it is now, firewalling the thrust levers if ground contact is imminent is a recommended Boeing procedure and thus practiced in the simulator with no apparent adverse indications.

Obviously it is better to damage engines than totally destroying the aircraft and kill all onboard no?
On the NG,There are no EGT temp protection inflight just N2 overspeed protection via the EEC and an overspeed governor.
I doubt the classic would be otherwise.
There is no mention in Boeing publications warning of this overtemp danger if the thrust levers are firewalled in an emergency
Pretty sure Boeing doesnt want pilots 'worrying' of possible overtemping the engines if firewall is necessary...
If in fact overtemping is likely to occur during a go-around, one would think this should be incorporated in simulator design?
It should not if your crews know the GA limit and do their job /sops properly and or use the AT arm mode..
I would think your airline trainers would have sufficient knowledge to teach/explain the negatives in firewalling 'systematically' and even over thrusting the thrust levers during standard go arounds...

I tried a discussion about this in thread about the autothrottle...but the temp /over thrust issue didnt seem to bother anyone at the time...

Good luck...

[lomapaseo]

There are limits and then there are really limits.

The real limits are defiuned in the limitations sections as regulated.

Thus the limit on mechanical speed is based on demonstrated min stress margins both centrifugal and vibration.

The temperature limits are based on time and temperature. Up to a certain level time is not important within the expected life cycle. Beyond this level time has an increasing bearing of accumulated hours or minutes.

Overboosting by command (within the other limits controled by the engine itself)is a rare contributor towards exceeding temperature and time to the point where a failure condition is imminent.

The greater risk is where an engine is allowed to deterioate by wear to a point where above average maximum temperatures have occured over thousands of flight cycles (unregulated and often untested)as in a case where maintenance is deferred until failure)

If you now combine such a deterioated engine with a commanded overboost (limited only by RPM) of course a failure may be expected. Fortunately the data is quite scarce in this regard

[Turbine D]

Hi Tee Emm,

de facto and lomapaseo gave you some good information regarding your questions and I am going to attempt to give you a good technical explanation.

Generally, there are 4 key thrust settings that are addressed by the power management control (PMC) system in the CFM56-3 engine:
1. TO - takeoff thrust
2. GA- go around thrust
3. MCL - maximum climb thrust
4. MCT - maximum continuous thrust

The takeoff rated thrust is the highest thrust rating given by CFMI for operational use by the aircraft manufacturer, in this case, Boeing. The thrust rating is based on maximum allowable EGT level at which the engine is allowed to operate at for 5 minutes. For a constant EGT value, engine thrust varies inversely with ambient temperature. The maximum EGT level is specified at sea level altitude and a specific ambient temperature that is higher than standard atmosphere conditions, usually between +27 to 36℉. This takes into account hot day aircraft performance requirements with no operating penalty. Now there are some exceptions to this.

Some airlines have negotiated increased takeoff and maximum continuous thrust levels to tailor an aircraft performance for high airport elevations or high ambient temperatures or both. This allows the airline customer to operate the engine at increased thrust and EGT levels and is often referred to as a bump rating. The earlier generation PMC's, as used for the CFM56-3, are analog electronic controls with input signals of N1, Power lever angle (PLA), fan inlet temperature (N2) and fan inlet static pressure. Since the fan and core engine is linked, the core speed is monitored and limited to prevent overspeed. There is a limited override of the core speed schedule in a limited PLA range, usually in the takeoff or maximum climb PLA regions. Fan speed is controlled as a function of T2 which is known as corrected fan speed. Effectively, the EGT is not limited, but only reported if it is exceeded.

When a new engine is final assembled, it is tested, among other things, to determine the EGT margin at takeoff conditions. The engine is guaranteed to have a certain % EGT margin to maximum allowed EGT and this testing confirms this. Then when the engine goes into service over time and cycles, the EGT margin decreases. This decrease is most often due to airfoil tip seals wearing in the compressor, HP and LP turbines, but other factors such as combustor flame pattern changes, HP air-cooled turbine airfoils becoming less effectively cooled, etc. So this degrading EGT margin becomes most apparent during high power settings, MCL, MCT, GA and particularly on those engines that have "bump" ratings. What is generally experienced is what I call an EGT overshoot.

When this happens, the engine does require an examination, generally a boroscope inspection of the compressor and HP & LP turbines to make sure the components are within operating conditions. Obviously, this makes airline management unhappy because of scheduling and aircraft utilization downtime that is experienced. However, in most cases, no damage is done by the EGT overshoot except for the fact it adds to the deteriorating EGT margin where at a point in time the engine must be removed and fully serviced. Certainly one would not want to limit thrust in a critical situation because of a momentary EGT excursion. OTOH, it would be desirable not to use high thrust settings for situations when it is not needed or just because it is available.

I hope this helps answer your question regarding EGT overshoots.

TD

[Tee Emm]

TD.
I am most grateful for the excellent explanation. It helped a lot in my understanding of the operation. In the simulator we see occasional "firewalling" in error during manual throttle take off's but especially on manual thrust go-arounds. It might be only be for a few seconds at a time before the PNF can get his hands on the throttles to ease them back toward the correct go-around thrust position - but it happens all the same.

I may have to actuate the instructor simulator button that gives a varying EGT (I will check if there is one in our simulator) and perhaps do that during a go-around to see if an indicated overtemp occurs and if the crew see it happening and fix it quickly. Otherwise it is all to easy for crews to become complacent and not closely monitor peak EGT in the simulator because normally overtemp doesn't occur in the simulator at the max thrust position.
Thanks for the other replies which have been most useful.
TM

[lomapaseo]

I may have to actuate the instructor simulator button that gives a varying EGT (I will check if there is one in our simulator) and perhaps do that during a go-around to see if an indicated overtemp occurs and if the crew see it happening and fix it quickly. Otherwise it is all to easy for crews to become complacent and not closely monitor peak EGT in the simulator because normally overtemp doesn't occur in the simulator at the max thrust position.
Thanks for the other replies which have been most useful.
TM

I would be most interested in what response you would expect from the crew in a go-around.

This has been debated before at much higher levels of rulemaking and the consensus was that the crew must fly the aircraft first and foremost and the engine(s) are expected to still get you back on the ground even in a damaged state.

In my book this is not a problem that needs fixing.

Do however pay attention to engine surging (booms, bangs pops) Those are time limiting during a go-around

[Turbine D]

ASFKAP,
There is protection against N1/N2 overspeed on the B737 NG via the FADEC, the B737 Classic does not have such protection and overspeeds are possible and indeed probable in the scenario described in the original post.
Actually the CFM56-3 engines on the Classic do have protections of both core and fan. These engines have a N1 speed governing system contained in the analog PMC. Scheduling of N1K (N1 corrected) is set by PLA biased by inlet static pressure (PS12) and limited by inlet total temperature (T12). I have a block diagram of this PMC. The main output signal is a feedback error signal to the hydromechanical N2 control torque motor to trim the N2 demand. I gave the inputs signals in my original post which were N1, PLA, fan inlet temperature and fan inlet static pressure. This PMC does have but limited interface with the aircraft systems except for on/off function, PMC inoperative indication and N1 demand signal to the cockpit. The PMC also maintains a full function hydromechanical core speed control which governs core speed, schedules transient fuel flow and schedules engine variable geometry.

Now as you point out, the CFM56-7 engines on the NG are digital designs with much more sophisticated power management architecture, receiving quite a few more inputs and linking to aircraft systems which provide for more outputs.

I agree that you can indeed experience EGT overshoot on the CFM56-3 under certain conditions and reduced EGT margin as it is an engine reportable function not a controlling function per-se.

Regards,

TD

[de facto]

There is protection against N1/N2 overspeed on the B737 NG

Not entirely correct....N1 overspeed protection yes but with AT ON only....N2 protection with AT ON AND OFF.

To avoid over thrust with the AT OFf, the Ng has a great feature which is predictive thrust ,as you advance the thrust levers you will see on the N1 display the N1 you will get for the TLA position...therefore allowing crews to correct a possible overshoot before the ACTUAL over thrust occurs.

[sheppey]

To avoid over thrust with the AT OFf, the Ng has a great feature which is predictive thrust

The 737 Classics should also have a great feature to prevent over-thrust and that is the presence of two switched-on pilots watching carefully. But sometimes neither pilot is switched-on.:ugh:

[aerobat77]

i do not have 737 experience, but afaik the cfm56 are flat rated to ISA+15 , so on temperatures below that they do not temp out at rated power , pushing the levers to the stop ( beyond max permitted thrust) will result in asking them for a higher thrust than they are approved for and they may or may not go in overtemperature or/ and n2 overspeed -depending on the outside temperature and air density . on cooler days the may just work outside its envelope without bursting redlines.

i remember a similar procedure on the bae146 where a windshear called for immediate go around power- when at go around power aircraft still sinks and ground contact is likely firewall the thrust levers. after this the checklist calls for a landing as soon as practical and a hot section inspection.

the bae engines are very likely then to go in overtemp since they are only rated to ISA - at all temperatures and elevations above isa they are temperature limited.

it seems that the fuel stop position with levers full forward is adjusted high enough to force the engine beyond certification limits but low enough that the manufacturer "believes" it should not result in an immediate destruction and may be so a final and last option to intentionally overload and hurt the engines but save the aircraft.

its pretty the same like overtorquing a turboprop - it may make it, but nobody will quarantee it since its neither officially tested nor approved.

fadec controlled engines give us the peace of not fearing an overshoot but on the other hand they do not offer us such a "last option" .

cheers !