Speed hold (auto throttle) disengages at 100 ft during landing

  • After the recent update, I noticed that in all the jetliners the auto pilot disengages the speed hold just before landing (~100 ft elev.). It used to be that it disengaged at touchdown, which I preferred. Not being a pilot, I'm curious which is the "real" way.

    Also, this wouldn't be a problem except that the plane assumes my throttle is at 0% after disengaging, even if I pre-set my joystick lever somewhere in the middle or at 100%, causing me to lose my glide slope and hit the ground hard. Annoying after getting used to the old way with nice smooth landings.

    Any suggestions? Maybe I'm doing something wrong?


  • Not being a pilot, I'm curious which is the "real" way.

    In the real world it's aircraft dependent. Traditionally, if auto pilot is engaged(auto land capable), the throttles are engaged through touch down. If Auto pilot is off, the throttles are disengaged. This is mainly Boeing types due to the pitching moments produced when thrust is changed. When I flew DC-10-30s, we didn't have autoland on our jets, but was allowed to land with throttles engaged. They would just retard to idle at 50ft. Though you could do that, most guys turned both off. In the GV and G550 that I currently fly, you can leave them engaged also through landing when manually flying, but no one does it. They just follow the traditional way of all on or all off. Most guys will kick off autopilot and throttles at 1000ft clear weather or 100ft to 200ft with a low ceiling. If the plane is not certified for autoland, you should be taking it off at the aircraft's min altitude for auto pilot. 300ft to 1000ft is a good place to kick them off since it gives you a chance to warm your hands up and get a feel before landing.

  • I can't speak for Airbuses, never flew them. Aircraft with low wing engines become problematic when the autothrottle starts to cycle. In the DC10, the throttles would go to full power or full idle if you were off selected speed by 5 KTS. As the engines surge back and forth, you are constantly fighting the pitching moment induced. So when hand flying, you end up working harder. For this reason, we had techniques such as 10% of your gross weight plus 23 for power. If I was 400K, 40 + 23 is 63% N1 for a 3 degree glide with flaps 35. I set 63 to 65% and the jet would naturally hold the approach speed. When I flew C-141Bs, we used fuel flow. We use fuel flow in the Gulfstreams I fly also. The only time I would keep autothrottles on with autopilot off was doing circles. The DC10 had a CWS mode between on and off with the AP lever. During the circle, the plane would hold what ever bank I input and allowed me to keep altitude hold. This way I could be outside more and not worry much about altitude and speed. Once ready for descent, click it all off and maneuver for landing. I know some companies were directing autothrottle use due to fuel savings. In this case, guys/gals were leaving it all on til a point at which they can take it all off. I do not like landing with autothrottles on while manually flying because it targets a hard AGL and rate of decrease. When flying heavies with a wide landing range, the flare and power pull is different at different weights. In the KC/DC10, it started at 50ft and had a slow rate. If you were heavy, it's fine, but leads to floating since the power is slowing coming off. The jet is heavy and has alot of momentum, you need to get the power off to prevent floating. If the jet is light, it responds quickly to power changes. This leads to firmer touch downs. Both of these causes issues and makes you flare differently from standard.

    In the G5/G550, it starts at 50ft and has a slow pull. Problem here is that you get excessive floating. In the G5, throttles need to be at idle at 50ft. It's very slick and loses the plus 5kts slowly. In the G550, you have to be at idle at 100ft. Landing with autothrottles on will completely ruin your landing. Do it on a short runway and you will have a career altering event.

    The 717 actually recommends leaving autothrottles on even when flying manually. Rear mounted engines don't throw of the pitch as much. Here is an exert from our BBJ(737) manual.

    Autothrottle use is recommended during takeoff and climb in either automatic or

    manual flight. During all other phases of flight, autothrottle use is recommended

    only when the autopilot is engaged in CMD.