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Warning: SLF query re engine sync

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Warning: SLF query re engine sync

Old 28th Mar 2011, 17:42
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Warning: SLF query re engine sync

The past couple of rides on AA TATL 777's I've observed periods of very noticeable low frequency vibration that I'd guess would be the beat frequency between the two engines. Sometimes it's constant for minutes, other times varies in amplitude with a short period (a few seconds), other times it's not noticeable at all. At its strongest it can be felt easily through well-padded seats.

I suppose it's been happening for years and I've just noticed, but given the search for "shareholder value" at many corporations, I wonder if tolerance on some maintenance step has been increased. (Still well within limits, of course.) And of course the underlying question would be what all that energy does for the airframe longevity.

pj
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Old 28th Mar 2011, 17:45
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I don't know about 777s but I spent many fruitless minutes trying to eliminate a 'beat' on 737s - the problem is that, unlike a piston aircraft, you have more than one rotating body running at a different speed in each engine and altering one affects the other.
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Old 29th Mar 2011, 17:58
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Three-shaft Rollers are easy to 'sync', two shaft models from other manufacturers, less so.
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Old 29th Mar 2011, 18:33
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Noticed exactly this on am EK 773 DXB-BHX a few days ago (along with the usual invasive start-up vibration from those massive donks).
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Old 30th Mar 2011, 02:58
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Syncronising engines in modern aircraft such as the B777 is usually a function of the FADEC (Full Authority Digital Engine Control) and the benefits are less vibration and consequently less fatigue on the structure and a quieter environment for the passengers.

However, the systems needs the engines to be reasonably matched and if one engine is changed, the parameters can be very different with the new engine being much more efficient. This is when the engines can can get out of phase with the FADEC having a hard time balancing the mismatched engines.

Older types without digital engine control need manual synchronisation. One type I flew had a 'Syncrotac' which had three little wheelts for engines 2,3 and 4. If all wheels were staionary, these three engines were sychronised with engine 1. Turboprops have a bigger problem with the propellers being larger rotaing masses. Analogue sycnronising systems were common from an ealier age, but again these were unreliable and often needed manual intervention.

Start up vibration is common. this is because the large fan baledes are slightly loose in their sockets. They rely on the enormous centripetal force when the fan is up to speed to hold them in position and give the fan it's rigidity. Before the fan is up to speed during start, one or more blade may be slightly out of position leading to the vibration.
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Old 30th Mar 2011, 13:52
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Syncronising engines in modern aircraft such as the B777 is usually a function of the FADEC (Full Authority Digital Engine Control) and the benefits are less vibration and consequently less fatigue on the structure and a quieter environment for the passengers
Does anybody have access to any words from Boeing or Airbus confirming this ?
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Old 30th Mar 2011, 15:57
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Throttle stagger or lack of it is a function of FADEC equipped engines, never heard of a synchronous function but would be interested to find out.

I thought any damage critical vibration prevention is better than cure e.g vibration trim margins lower, and resonance flutter margins/ranges kept lower, V2500 bus drivers will understand, the engine races through the flutter range when going through the quadrant up to max climb.

Got the heart thumping the first time I did a run on one!!
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Old 30th Mar 2011, 16:15
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Found the following:

The 717 autoflight system is adapted from the MD-11 system. It consists of two flight control computers (FCC) and a glareshield control panel (GCP). The FCCs provide autopilot, autothrottle, flight directors, stall warning, wind-shear detection and guidance, engine synchronization, and various other functions

And from Airliners.net regarding 737-700/800:

Below are some extracts from the SDS.

Display Electronics Units (DEU) The A/T computer sends mode data to the DEUs to show A/T modes of operation on the FMA on the CDS.
The FMC calculates engine N1 limits and N1 targets
during each flight phase and sends the data to the DEUs. The DEUs show N1 limits on the engine display. The DEUs send N1 targets to the EECs.

FMC The FMC calculates thrust N1 limits and N1 targets for each flight phase. The data goes to the DEUs. The DEUs show the N1 limits on the engine display. The DEUs send the N1 targets to the EECs which calculate equivalent TRA targets to send to the A/T to set thrust. The FMC also sends N1 targets directly to the A/T. During takeoff and max thrust go-around, the A/T uses EEC TRA targets and FMC N1 targets to set thrust. During takeoff, climb, and max thrust go-around, the FMC N1 targets are the same as the N1 limits. During reduced thrust climb and cruise operations, the FMC N1 targets are less than the N1 limits.
EEC The DEUs send FMC N1 targets to the EECs. The EECs use the data to calculate equivalent TRA targets. The A/T uses the EEC TRA targets to set thrust during takeoff, climb, and max thrust go-around. For takeoff and max thrust go-around, the A/T initially uses EEC TRA targets to advance the T/Ls. As the T/Ls get to within 4 to 6 degrees of the FMC N1 limit, the A/T computer then uses FMC N1 targets to make final T/L adjustments to the FMC N1 limit.
Flight Management Computer (FMC) The FMCs send this data to the A/T computer: N1 targets Gross weight Minimum speed FMC altitude Static air temperature FMC modes GMT/Date.
BITE test information. The autothrottle computer converts the target N1 values from the FMC to an equivalent TRA target. The target N1 rating is dependent on the FMC engaged mode. Gross weight is used in the go around control logic and approach control logic. Minimum airspeed is the lowest airspeed that is acceptable during VNAV operation. FMC altitude from the FMC is used for anticipation of altitude acquire during VNAV operation. SAT is used to calculate a backup TRA limit value. The FMC mode discretes are used to determine control gains and limits.
Electronic Engine Control (EEC) Each EEC channel sends this data to the A/T computer: Thrust resolver angle (TRA) N1 command indicated TRA for max forward idle Estimated corrected thrust TRA for actual N1 TRA for N1 target TRA for N1 max TRA for 5 degree/sec response. Thrust resolver angle is used by the autothrottle computer to calculate an N1 command. N1 command indicated is used to set a throttle position using the error between the target N1 (from the FMC) and the commanded N1 from the EEC.
TRA for maximum forward flat is the throttle angle below which the engine is in the idle range. Estimated corrected thrust is used in the reduced go around control logic. TRA for actual N1 and TRA for N1 target are used in the N1 mode control logic. TRA for N1 maximum is used as a reversion limit in the event that the airplane is dispatched without an operative FMC and as a protection from excessive throttle angles. TRA for 5 degree/sec response is used in the retard control logic.
So with autothrottle engaged N1 should be very closely matched. However the beat frequency heard will be a function of the tolerance allowed by the control system (there will necessarily be hysteresis in the system I assume or the RPM will be in a constant chase mode).

This may be more significant with a larger engine (like the 777 donk) I'm guessing without thinking too hard.

- GY
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Old 30th Mar 2011, 17:35
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727 - pilot sync (good luck with three engines, some days it was a bear)
S80 - pilot sync unless N1 SYNC switch engaged
A300 - pilot sync
757/767 - pilot sync
737-800 - electrons. Automatic, does an excellent job.
777 - ? Electrons or pilot?

I've always enjoyed the heavy vibration cycle that comes with power reduction for descent. If the pilot allows the N1's to get out of sync it's very noticeable in the mid 60's% N1.
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Old 30th Mar 2011, 21:08
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Older types without digital engine control need manual synchronisation. One type I flew had a 'Syncrotac' which had three little wheelts for engines 2,3 and 4. If all wheels were staionary, these three engines were sychronised with engine 1.
Nimrod MR2 has this. Did the Comet 4 on which that aeroplane is based have it? The Comet has Avons while the Nimrod has Speys (which are said to be turbofans, but to me they look like hi-bypass turbojets).
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Old 1st Apr 2011, 06:04
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How do you sync the N1s on engines which use EPR as the primary reference?

The EPR's may be equalised on FADEC aircraft with EPR as the primary parameter, but this does not mean the N1's are running at the same speed.

Rgds
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Old 1st Apr 2011, 06:13
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On the Douglas/Boeing 717 the EEC synchronises the Left EPR to the right during takeoff, and during climb, cruise & descent, the N1s.
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Old 1st Apr 2011, 06:54
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On the 744 the Autothrottles do the synch function. If the autothrottles are off (unusual, except on approach), there is no synch.

On the 747 Classic, the pilot or FE does it manually by ear/feel.
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Old 1st Apr 2011, 08:27
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Flying both GE & RR powered 777s, I've noticed that the GE engines appear to match N1 (rpm) but the RR does it on EPR which often leads to "phasing" vibration as the RR N1s don't correspond to an exact EPR value for individual engines, as NSEU points out...
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Old 1st Apr 2011, 10:04
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I also think that NSEU has hit the nail on the head. The primary requirement must be to match the thrust, not the RPM.

However, in the case of what is sometimes loosely referred to as an "N1 engine", such as GE and CFM, the thrust is considered to be a function of N1. EPR is not used. As the fan typically generates about 5/6 of the thrust, it's not a bad way of achieving the requirement, unless the fan of one of the engines is more worn than the other(s). The A/THR will choose the N1 it thinks appropriate, and order the FADEC of each engine to deliver it. So the N1s should be fairly well synchronised.

In the case of "EPR engines", such as RR, IAE and (previously) P&W, the A/THR must order all the FADECs to give the same EPR. If one of the engines is more worn than its neighbour(s), it will need a higher N1 (fan) RPM to achieve the ordered EPR.

Perhaps poorjohn will confirm that he is talking about RR engines?


Quote from Dan Winterland:
One type I flew had a 'Syncrotac' which had three little wheelts for engines 2,3 and 4. If all wheels were staionary, these three engines were sychronised with engine 1.

We had those on the HS114 (De Havilland Heron), to synchronise the four Gypsy Queen engines (each 250HP). Some years later, I was amused to find the identical instrument on the VC10, for the N2 spool of its 21000lb-thrust Conways. Being merely the copilot, the grizzled FEs didn't like me fiddling with the throttles in the climb and cruise (they had their own set), and were not impressed when I told them why I knew what I was doing.

Last edited by Chris Scott; 1st Apr 2011 at 10:36. Reason: Para 4 added. Removal of the expression "knackered"!
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Old 1st Apr 2011, 10:07
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How do you sync the N1s on engines which use EPR as the primary reference?
The answer is you don't. Its quite common on the V2500 Airbus to see both engines showing the same EPR but different N1s. Its not really a problem unless one engine is much older than the other meaning it has to go round faster (and burn more fuel, higher EGT) to produce the same EPR as the other one. This is what produces the out of sync sound. The only solution, if you can be bothered, is to disengage the autothrust, sync the engines and just except the very slight power difference
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Old 1st Apr 2011, 12:48
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Excellent thread gents!

For some reason EPR synch had not occurred to me. That would indeed explain the difference.

I'm not sure I understand exactly how
"the fan of one of the engines is more worn than the other(s)"
, thinking that it would be more likely that the combustion/power turbine stage would be the point where "wear" would introduce power loss, and hence imbalance the RPMs as a function of equalizing EPR.

- GY
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Old 1st Apr 2011, 14:35
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Quote from GarageYears; my inference added in square brackets:
"...it would be more likely that the combustion/power turbine stage [rather than the fan] would be the point where "wear" would introduce power loss, and hence imbalance the RPMs as a function of equalizing EPR."

If you are referring to the core of the engine, then I think you are right to point out that the higher pressure used for the engine pressure ratio (EPR) is sensed after the last LP turbine (I think!); not in the bypass duct or C-duct. Perhaps someone can confirm that?

In the line of my post you quoted, however, I was not talking about trying to equalise EPRs; I was talking about engines that use N1 as the primary indication of thrust:
"...in the case of what is sometimes loosely referred to as an 'N1 engine', such as GE and CFM, the thrust is considered to be a function of N1. EPR is not used. As the fan typically generates about 5/6 of the thrust, it's not a bad way of achieving the requirement, unless the fan of one of the engines is more worn than the other(s)."

In the case of engines that use EPR as the prime indicator, I argued that a worn engine would need a higher N1 than its brand-new neighbour to achieve the same EPR. On reflection, I think that both (or all three) shafts would be faster, as they interact with each other.

But the "low frequency vibration" and/or "beat frequency" that poorjohn notices is presumably from the N1 shafts.
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Old 1st Apr 2011, 18:42
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Scott, afaik AA uses Trent 800s.

The question behind my question was whether during the certification process engineers assumed a higher degree of syncronization and thus less vibration than the industry is achieving in the present economic climate.

I have no expertise but can imagine valid reasons for engine life being extended, thus setting up for bigger differences between a 'new' engine and one nearing end of (a longer) service period.

It's just as likely that nothing has changed and I suddenly noticed it. Probably no big deal if the airframe doesn't mind.

pj
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Old 1st Apr 2011, 19:56
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The so called beat frequency is sensed more by the human ear than the airframe.

This whole phenomena is about perceptions of the human ear and while likely associated with something spinning in the innards of the airframe it is very unlikely to be associated with causing any kind of damage to a critical aircraft structure.

I tend to think of the explanation of two disimilar drum heads sitting in the middle of the aisle of a plane. One begins to hum while the other doesn't and then when it stops then its neighbor starts and so on until they both become quiet. Bothersome to the ear but nothing else to be concerned about.
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