Very strange occurence during cruise. Anyone know cause?
 

A very strange occurence happened to me over Germany whilst flying a Seneca V in very stable conditions ( strong high pressure).

I was at FL50 and had been in the cruise (155kts IAS) for about 20mins. All power indications reading normally VMC and Autopilot on (HDG ALT FD) . All very calm just waiting to be changed over to another radar unit when suddenly the a/c rolled sharply to the right to about 45 degrees AOB:eek:. I immediately disengaged AP rolled wings level and pitched forward ( to reduce AOA in case it was a high speed stall). I also instantly checked engine T&Ps they were all reading normally. Then suddenly about 2 seconds later the a/c did the same thing:mad:, rolled quickly right at which point I quickly rolled wings level. Visually checked flying surfaces all ok and controls felt fine. Nothing else abnormal and from then on perfectly normal flight. To me two possible causes.

1) There was a C-130 about 8nm in our 12 o clock same heading and altitude so we were effectively follow his track. I feel the there could have been wake turbulence from this that could have caused 2 wing drops. However, surely wake turbulence will decrease in altitude once it has developed so if the C-130 was at our level and 8nm away then wouldn't his wake turbulence be beneath us and so not effect us? Also all we had only 2 wing drops there was no small buffets, turbulence or disturbance in our flight condition prior and after the event.

2) On the previous leg we had problems with the AP which when put into a rate 1 turn would seem to rock the wings relatively slowly but definatley pronouced manner. Could this same glitch with the AP cause a sudden wing drop? What puzzles me with this possible cause is that AP was disconnected before the 2nd wing drop. It has been mentioned to me that the AP doesn't fully disengage immediately so may have been having some effect, but I'm not sure about this.

The a/c is been taking to maintence shortly and definately won't be used with passengers until the problem or cause is found (luckily no passengers on this leg). Would anyone else know what could have been the cause here or have had a similar occurence? Any help or advice would be greatly appreciated.

Regards,

Mooneyboy:confused:

Sounds like wake turbulence, especially if your rolls were in the same direction.

Yes, while wake does descend below an aircraft, you can still certainly experience it at the same level while in trail.

Autopilot problems can do what you describe, engaged, or not. This is why emergency checklists generally include direction for electrically disabling the autopilot once it has been interrupted in a runaway situation or malfunction (usually by pulling the appropriate circuit breaker(s)).

Last winter we had a LR35 autopilot that kept engaging itself and taking the controls in the roll mode with no pilot input. If you have a known autopilot problem, completely disable it, and certainly don't go flying and using the autopilot.

During stable conditions the vortexes survives longer and changes altitude with less rate. This makes the vortex theory more likely than if it would have been in unstable air.

wake
 

as some have already put it, most likely it was wake turbulence
i was once flying a C-206 and my friend about 8 nm ahead of me in a cherokee 6-300 same level FL055, same track...smooth, cool, morning, cavok
it lasted about 10 secs, i lost almost all aileron control...ailerons became sluggish like the aircraft was stationary on ground
helpless, with my feet on rudder pedals i came out of it without wing drop(s)

Wake turbulence does not always decrease with altitude. We used to fly three of four aircraft in loose formation out to an island weekly. Often one aircraft (GA piston types) would be up to 5 miles behind the other due to ATC delays and sometimes would experience wake turbulence from the preceeding aircraft in cruise at the same level. One day we allowed one aircraft to catch up to the other (both on AP) to see the altitude difference - there was none.

If it was the autopilot the control wheel would have moved, if not - wake turbulence. Probably.

Thanks for all your replies. SNS3Guppy I did as you said and disconnected the autopilot by pulling the CB immediately after the second wing drop. Did not have a problem once I did this. Another thing which still makes me think it could be the autopilot is from what DB6 said. The control yoke did roll right both times. I remember it was quite a force to overcome it. This coupled with the problem we had with the rocking wings on the previous leg with the autopilot on makes me believe this could be the problem (bit of a coincidence).

Also very interesting what kme, lowbypass and compressor stall have said and its been interesting to read your own experiences. I did'nt realise that wake turbulence can still be experienced at the same level a fair distance behind so thankyou for sharing your knowledge on this. I will definately bear this in mind next time I am behind another aircraft at the same level.

The a/c will be going into maintence very soon so I will let you know their findings and once again thanks for your replies.

All the best,

Mooneyboy:ok:

This is some info i had passed onto me about W/T.

If the generating A/C is say an A380-800(HEAVY)
Following A/C----------------Min. Seperation
A380-800---------------------4 nmi
Non-A380-800 Heavy--------6 nmi
Medium-----------------------8 nmi(MTOW of greater than 7,000 kg)
Light--------------------------10 nmi (MTOW of 7,000 kg or less)

Suprising enough there are 3 sections to W/T, all pose a threat.
Immediately behind the wake-generating aircraft is a region of wake turbulence known as the roll-up region,Farther away from the generating aircraft is an area of the wake known as the plateau region, where the vortices have merged and/or attained a nearly constant structure. Even farther downstream from the generating aircraft is a wake area known as the decay region, where substantial diffusion and decay of the vortices occur due to viscous and turbulence effects. At altitude, vortices sink at a rate of 298 to 498 FT per minute and stabilize about 498 to 898 FT below the flight level of the generating aircraft. In some atmospheric condition, they may stay stable but this is only found close to the ground, so you'd most likely experienced Wake Turbulence at the same FL 8nm behind the generating A/C ... even though this isn't a highly likely situation, it was more tha likely the atmospheric conditions aided in keeping the W/T at the same FL and in your path, just a freak situation i guess.
hope this info helps, I know it helped me understand W/T a bit better when it was passed into me. :ok:

Thanks jetflite, interesting what you have written especially about wing vortices stabalizing 498ft to 898 below the flightpath.

The a/c has come back from maintence no obvious problem was found with the AP but it was only a prelimary check. This and your suggestions make me believe that it probaly was wake turbulence. All though saying this the a/c still rocks its wings on AP so it will have a more extensive examination shortly.

Once again thanks for all replies,

Mooneyboy