I was following Air Canada flight AC872 from CYYZ to EDDF last night on FlightRadar24. After takeoff, it climbed directly to 35,000 ft. It passed Newfoundland at about 0100 UTC this morning on N181F but, as it crossed waypoint JOOPY at the entry to NATU, it descended to 31,000 ft before going beyond radar coverage. When it reappeared at the other side, it was back at 35,000 ft.

 

What could be the reasons for descending? As far as I could see, it was the only aircraft to do so.

 

Likely for separation from preceding traffic prior to entering the non-radar environment. Pilot request due to turbulence avoidance could be another reason. The climb to FL350 on the other side could be explained by re-entering the radar environment.

 

 

Les Parson

I guess it must be the separation. The flight in front was at 35,000 ft, too, and maybe it was closer than I realised. I had thought that the flight level should be established well before the NAT entry point. The flight in front came out the other side at 39,000 ft so I suppose that's why the AC872 could return to 35,000 ft.

 

I can't imagine a pilot descending because of turbulence.

Depending on the level of turbulence, it makes perfect sense to descend, even at the expense of added fuel burn.   On my return trip from a vacation to San Jose California a month ago, the captain of the AA MD-83 mentioned to us that we would be flying the entire trip to KDFW at FL270, just to avoid chop.    Yes, they burn more fuel, but it beats having to send passengers to the hospital upon landing at the destination or even at an alternate airport.

I can't imagine a pilot descending because of turbulence.

If it is bumpy, you go wherever the smoother air is, higher or lower. Often, lower may be the only option since we may be too heavy for the next higher flight level for the direction.

Thanks for all the responses. What conditions would cause turbulence only at higher altitudes? I assume that turbulence was not a factor in this particular case because the flight just ahead of AC872 remained at 35,000 ft.

Turbulence can occur at any altitude, not just 'higher' altitude. It occurs for any number of reasons, such as jetstream boundaries, frontal areas, differing winds aloft layers, clouds, convective activity, etc. It all pretty much eventually leads back to uneven heating of the planet's surface by the sun.

When I was thinking of atmospheric turbulence, I was thinking only of direct convection of warm air from the ground up causing turbulence at all altitudes, in which case one would either climb to a higher altitude to go above the turbulence, or divert to go around it.