407 Exceedance Mystery

[Gmach]

We have had 2 unusual and unlilely exceedances recorded on our 407 in the past couple of months. One was for a very high torgue reading that in fact could not have occured and on a seperate occasion an MGT exceedance recorded during cruise, while well within limits. Has anybody else experienced any phantom or unexplained exceedances recorded on the 407? This could becoming a rather expensive problem. Bell, of course, has no explanation. "...very odd..." was their reply when questioned about it!

[BlenderPilot]

I have flown close to 30 different 407's, worked in a Bell Helicopter Service Facility, have seen a millon different weird things, but, NEVER A MYSTERIOUS EXCEEDANCE.

Why do you say the exceedances "couldn't have happened". The ECU records all the parameters at the time of failure, for example what did the N1, MGT, do while the overtorque occured? in the graphs that you downloaded from the ECU, did they move at all??

I have seen several people exceed MGT in cruise flight, for just not paying attention and flying too close to the yellow for too long, so it's not impossible.

[vaqueroaero]

I have met one person who has had a mystery exceedance. I don't remember what it was, but according to the rest of the data it was not possible, because there was no spike in any other reading.

It's very rare - maybe that person was you.........!

[Graviman]

Gmach, do you fly in gusty conditions? Blue sky thermals can be suprisingly strong.

[Aesir]

I know a pilot who had a unexplained exceedance in a AS350B3. The recurrent exceedance recorded very high T4 in normal cruise flight at 80% FLI.

Eurocopter had no explanation but the company changed a temp sensor and it never happened again.

[ShyTorque]

Gmach, I've sent you a PM.

[Shawn Coyle]

The Bell 407 FADEC will automatically start a record if there is an exceedance. it will grab the previous 10 seconds worth of data, and record for another 50 seconds. It should then be downloaded after landing and the whole thing looked at.
A mysterious exceedance would only show up on one channel (say torque) and not have any corresponding N1, TOT, fuel flow, etc to confirm there really was that much power being put out.
But be careful - the FADEC data only records data every 1.2 seconds!

[Simul8]

Do you, by chance, have the Reversionary Governor ECU installed in your A/C? ECU-35R?

[havoc]

Maybe not the same issue: friends at PHI EMS said they are downloading and clearing the ECU data weekly. I will get the AD and Rolls Royce #s but it appears that PHI has seen accumulative occurences that at a point equals an exceedence. The base that this occurred at was 107% N1 with no other exceedences. As I said not sure if this is related.

Not PHI but our acft on second and third starts of the day have had delayed lightoff 16% or FADEC aborts at 320 MGT or 20% N1. We have an HMU on order. Computers downloads shows nothing going on.

[spencer17]

@Gmach: I had this in an EC120 flying thru the beam of a very strong transmitter station. There where exceedences in torque and rotor rpm that, in real, would be impossible. R-RPM + 100.

Happy Landings
Spencer17

[Shawn Coyle]

The ECU is supposed to be immune from HIRF (High Intensity Radiated Fields). If you encounter something like this when flying close to a high-power transmitter, you should be filing a Service Difficulty Report, or something to that effect with both the manufacturer and the civil aviation authority.
If you don't, nothing will ever get done to fix the problem.

[Brilliant Stuff]

All I got was Russian in my headset but then again it was only a R22.:}

[VRS]

I've just had one of my own it seems (suspected Ng exceedance according to data) It appears to have occurred on standard climb out profile (fully loaded) using 5 min limit MGT (approx 740 degC for less than 2 mins) given MGT has always been the overriding FLI in these hot/high/humid conditions. i.e; the MGT will generally not allow you to pull enough power to exceed the NG, and if you did, you would most definitely have exceeded MGT limits...
I'd been doing the exact same T/O profile in exact same conditions in this and other fleet 407's for the past 8 months without issue, which is why it came as a surprise. If anything, I would've expected an MGT exceedance over an Ng issue.:confused:
The proof will be in the pudding tomorrow I guess when the data is extracted by engineering, to see if I was a naughty boy and heavy-handed, but I can almost guarantee there was no spike in the 2 other parameters (MGT/TQ) as I had already buttoned the power demand - from the initial lift and roll - off into a steady climb.
I notice this thread has not been added to since 2007. Any other occurrences of interest out there?

[cattletruck]

Hopefully the same log data that illustrated an exceedance will also illustrate a failing sensor or dial when correlated with other input data. That's how it works with digital systems, although the jury is still out whether they are better than current analogue systems, but in time with further developments they probably will be.

[VRS]

Confirmed...
As determined by engineering, it has turned out to be an electrical failure.
Interestingly, this occurrence is NOT so rare, as we had an almost identical incident a few years back (faulty gauge)
A theory could be; these digitally operated systems simply don't like wet/humid environments. Development then has the challenge of designing a system that can cope better in these conditions.
This has been another one of those interesting learning curves and hopefully this thread may help out the next guy/girl who experiences a similar occurrence.
Something else I'll take out of this; I think by drawing back on our training, sticking to the basics and maintaining a habit of flying any helicopter (analogue or digital) within it's limits, we have a pretty strong case to suspect something might be amiss, even if it might be VERY out of the ordinary. Then it comes down to troubleshooting.
Good old fashioned "Seat of the pants" vs "the computer" :ok:
Safe flying!

[Phoinix]

@Gmach: I had this in an EC120 flying thru the beam of a very strong transmitter station. There where exceedences in torque and rotor rpm that, in real, would be impossible. R-RPM + 100.

Happy Landings
Spencer17

Both EC120 I fly have about 1 RRPM exceedence in 30 flights that is not possible; like RRPM between 490 and 540. The maintenance had contacted ECF and they replied that they are aware of the bug... but that was it.

[NickLappos]

It is likely that a fast "spike" of data, caused by a sensor or electrical glitch, is to blame. The best way to answer the question is to dump the data as a time history, and look for the exceedance as a spike that is not consistent with the rest of the data. For example, the torque jumps up in a fraction of a second, but the collective pitch does not, and the altitude and load factor doesn't change. This quick study is proof positive that no actual exceedance took place, and the only maintenance action would be to solve the erratic reading.


I had such an occurrence back 30 years ago in an S-61, where the Time-Temp recorder (a new installation of the type) flagged a massive engine overtemp, and the "exceedance police" came to cut off my head. It turned out that a voltage glitch on shutdown created a mythical temp spike that was flagged.