On our return from Europe this week, I was so bored that I found myself listening to the engines on our Airbus while at cruise altitude. It seemed that they would, for want of a better way of describing it, "spool up" for a few seconds, then "spool back down" again. This cycle seemed to repeat around once every 8-15 seconds. We were not changing altitude, nor did the flight display suggest that we were accelerating/decelerating either. And we were in super-smooth air. Any idea why it was behaving this way?

Sounds as though the Auto-Throttle was out of calibration and "hunting."

Solution: Disconnect AT.

+1 to what Dick says above. Found it quite common on the planes I've flown.

Was the sound coming from the engines or was it the overhead environmental air blowing in? I've heard the air cycle machines cycle up and down in the cabin and it's pretty noticeable.

Got any ideal what type engines? I flew GE's. Some are equipped with Rolls Royce. What type A/C and carrier?

Mike

It was an Airbus 310. Definitely the engines, not the environmental system.

Not sure about the 310, but on the A319/320, that scenario occasionally occurs in cruise when in "soft altitude hold" and the plane wouldn't maintain the target altitude.

For VNAV operations (where the FMS controls the speed instead of the Flight Guidance Computer), the autopilot will climb or descent up to 100 ft in order to maintain the selected mach. This alleviates the need for the autothrottle system to chase the desired mach. If, however, it would take more than 100 feet to recapture the desired mach, the autothrottle system would switch to speed mode and spool up the engines to recapture the mach while the autopilot regained the target altitude. So, on some planes, every 10 or 15 minutes or so the engines would spool up to regain the lost energy. On most of the planes, however, the 'soft altitude hold' was adequate and the engines never changed target EPR or N1.

Doug