AP Descent Very Subtle Porpoise During Decent (777-200)

[Sinistar]

Okay porpoise maybe the wrong term.

On a long boring flight home from vacation I had the seat entertainment showing the HUD like view during the start of descent from around FL340. After about a minute or so I think I noticed a pattern where the descent would first be like 20ft every half second for about 200ft and then it would seem to slow down to like 20ft every 1 to 2 seconds for another 100ft or so and then back to the faster rate. I would think in something as automated and expensive as widebody over ocean plane would be a perfect rate.

Obviously no big deal and talking very tine variations of 20...60ft per second over 34,000 ft. I was wondering if this a actually a case of the plane doing what it is supposed to do? The AP commands some minor amount of elevator the descent rate increases the airspeed increases and the tail starts to generate a bit more lift causing the decent rate to slow - or something like that?:

 

[Skyrys62]

I often feel oscillations like that too coming back from vacations.

 

[bflynn]

Google phugoid oscillation. I don't usually notice them in the airlines.

 

[Tarheelpilot]

It’s a Boeing.

 

[Sluggo63]

Okay porpoise maybe the wrong term.

On a long boring flight home from vacation I had the seat entertainment showing the HUD like view during the start of descent from around FL340. After about a minute or so I think I noticed a pattern where the descent would first be like 20ft every half second for about 200ft and then it would seem to slow down to like 20ft every 1 to 2 seconds for another 100ft or so and then back to the faster rate. I would think in something as automated and expensive as widebody over ocean plane would be a perfect rate.

Obviously no big deal and talking very tine variations of 20...60ft per second over 34,000 ft. I was wondering if this a actually a case of the plane doing what it is supposed to do? The AP commands some minor amount of elevator the descent rate increases the airspeed increases and the tail starts to generate a bit more lift causing the decent rate to slow - or something like that?:

777 guy here.

First, I'd say that I don't know if I'd trust the IFE to give you a true representation of the the VVI is at any point in the descent. Did you feel that the airplane was oscillating like that, or were you just going off the IFE altitude display?

Next, there are several different descent modes of descending in the 777 (most modern airliners will be very similar). The two most likely descent modes happen in VNAV (Vertical NAVigation). You'd most likely be in VNAV PATH or VNAV SPEED. VNAV PATH is when the plane is descending on a specified vertical path, and the speed is being controlled by the thrust levers (it's trying to do an idle descent on path). This mode would more than likely not cause the plane to porpoise like you're describing. The other mode, VNAV SPEED, typically happens when ATC tells you to start down early, before the computed TOD (Top of Descent). On it's own, the plane will descend at around 1,200 fpm and a programmed speed, and will descend with "speed on pitch" meaning that the plane will pitch up or down to maintain the selected descent speed. This is where you'll get some oscillation. The 777 and most other airliner autoflight systems are programmed to be more than anything, smooth. You don't want those first class passengers spilling their champagne, do you? So, as the plane is descending in VNAV SPEED the speed kind of free-floats. Speed may build a few knots while descending, then the autopilot will smoothly pitch the nose up to get back to the programmed descent speed. Now, with the autopilot not wanting to do anything abrupt, it may overshoot that speed and get a few knots slow. It'll then lower the nose to get the speed back. This is probably what you're noticing. I'd say the frequency is a little longer than the one minute that you're describing, but I wouldn't be surprised either. Especially if the pilots are adjusting the thrust levers during the descent.

That's one of the more annoying aspects of Boeing's VNAV system. McDonnell Douglas's PROF in the MD-11 was so much better. To avoid that, I typically descend in VERT SPEED instead of VNAV SPEED which allows me to set a specific vertical speed AND descent speed and the plane will fly those two parameters without pitch oscillations. And I don't even have passengers in the back to worry about.