Automatics Response

[Chris the Robot]

It's probably quite a technical thread but since I was a passenger at the time and I haven't got a licence yet I thought I'd post here.

On 31/10 I was on an Embraer 195 inbound to BHX when what appeared to be a crosswind (with some windshear) was encountered on late descent. The aircraft banked repeatedly left and right to an angle of about 10 degrees each way with a relatively constant frequency of about 0.1 Hz.

Also, on some aircraft (both Embraer 195 and Boeing 717) I have noticed that on final approach, rather than staying at a constant power the engine power moves between what sounds like 35-60% N1 again with a frequency of about 0.1 Hz.

During this time I assume the autopilot/autothrottle is engaged due to company requirements etc. I've thought about why the autopilot may perform as above and the only solutions I could come up with were fast response time/under damping and the fact that the automatics aren't "intelligent" in the same way as humans. Are the any other reasons why an autopilot behave as above?

[eastern wiseguy]

No Idea...try asking on the TECH LOG thread.

[Hotel Tango]

To be honest, I don't think they will no what he's on about. Tip: if you're a layman ask your question in layman terms.

[Schroedinger]

That's 600 times per second!

[DaveReidUK]

That's 600 times per second!

I think not.

[reportyourlevel]

Hertz is cycles per second, so 0.1Hz is a tenth of a cycle per second or one cycle every ten seconds.

[Chris the Robot]

Its probably a good idea for me to clarify a few things here.

The Embraer was quite late during the descent but not on final approach. There was wing bank but no turning, the aircraft was I believe flying a reasonably straight course at the time. I was seated in an aisle seat and out of the window was repeatedly seeing only sky then only ground over and over.

What I was saying regarding power was that on some aircraft on which I have flown as a passenger, the engines sound as though they are increasing and decreasing in power by quite a bit in a cyclical fashion.

Fast response and under damping are mechatronics concepts. A fast response is where a system reaches its desired output quickly following a command. Under damping is where a system overshoots its desired setting before trying to "chase" it. I was trying to say that maybe the automatic systems were overshooting their desired parameters both ways thus "chasing" them. I was wondering if anyone knew of a reason why the autopilot did this. Without wanting to open up an "automatics vs. hand flying" debate I was wondering if this was where a human may be better than a computer, flying the aircraft without chasing the airspeed or other parameters.

Finally, I believe that only moderators can change the location of threads, so if people think this is better in the Tech Log section, could it please be moved?

[PAXboy]

Chris the Robot If you edit your first post and use the delete 'button' the whole thread will go. Yuo can they rephrase your question in Tech Log. Do bear in mind not to be TOO outspoken in what you THINK was happening to the a/c! It's not easy to tell from the cabin, even for an experienced pilot.

[Chris the Robot]

Thanks for the advice PAXboy, I'm going to delete this thread and place the same question (re-worded) into the Tech Log.

[Hand Solo]

I doubt that the engines would be moving within such a broad N1 range, that would be some major power changes. I suspect if you are getting a regular 'beating' sound effect it's probably to do with the two engines operating at slighty different speeds. This normally affects prop driven aircraft when the props aren't in sync but I've heard it on turbojets too. Autopilots can respond in funny ways and get into small oscillations but they usually damp themselves out withing a cycle or two (in fact I think it's a certification requirement). Pilots also cause oscillations. I wouldn't assume that the autopilot/autothrottle was engaged. In gusty conditions it's sometimes better to take them out. The A319 in particular can be a real handful when lightweight in gusts and manually flying it produces better results. IMHO.

[Chris the Robot]

Thanks for the info on that Hand Solo, I'll keep the thread now but post all future questions of this sort in the Tech Log section.

[Piltdown Man]

On the E190 the autothrottle is a bit 'binary' in operation, but damped in its response. Therefore is regularly under/overshoots - and follows this with a correction. The ailerons are very powerful but to prevent excessive use, they have a very high, zero biased, spring "feel" force with no aerodynamic feedback. In my opinion, they are totally un-harmonised with the other controls. This gives a "rock'n'roll" ride in blustery conditions.

PM