Originally Posted By: PimTac
Mentioning the 757 reminded me of a flight I was on a few years ago out of Hong Kong. This was not United but Cathay Pacific. The aircraft took off and climbed at a pretty good angle. Having flown out of there a number of times this was new to me. That 757 was like a rocket. At around 3000 feet ( not sure exactly) the captain pulled back on the throttles and we experienced the sensation of stopping in mid air. It was very noticeable and then it started to feel like the plane was falling backwards. A few passengers screamed. One young girl a couple of rows ahead of us was crying.
That sudden change in forward motion was a very weird sensation. I wonder if that maneuver was as planned or unexpected for the flight deck crew as well?
OK - I'm pretty certain that I know what happened, but the explanation is technical, so bear with me...
Depending on weight, wind, runway length, terrain, weather, noise-abatement requirements, etc. we may use full power for takeoff. If we do, in a 757, it's pretty sporty. The airplane has great thrust/weight for an airliner.
We used to do this all the time in Orange County, CA, and in fact, because it was dramatic, we were required to brief the passengers that the takeoff would be steep, followed by a substantial power reduction at level-off. And that always was dramatic. A 20 - 22 degree nose up climb followed by a nose-over level-off and big power reduction.
Hong Kong, whether the old Kai-Tek, or the new Chep Lap Kok, doesn't really have noise abatement concerns, so I don't think it was that in your experience.
The Flight Management System (FMS) on the 757/767 drives the flight director and/or autopilot as well as the autothrottles. In a full power take off with a low level level-off, the system can get pretty agressive. There are a couple of modes that manage pitch and power, one is VNAV, one is Flight Level Change, we'll skip the other.
VNAV (Vertical Navigation) manages pitch and power to comply with vertical crossing restrictions, published speed restrictions and fuel vs. time climb speeds. Pretty sophisticated interface between navigation and managing fuel. Flight Level Change is simple: full power (or idle if descending) to get to the altitude desired using a manually selected speed.
What can happen is this: as the FMS responds to the rate of climb, which is high in a full-power takeoff, it "captures" the altitude early...it's judging how quickly it can lower the nose from a steep climb, either via Flight Director guidance or using the autopilot. In a low-altitude level off, the FMS is still using V2+15 knots as its reference speed (so, a much slower speed than climb speed, call it 170 knots, or something similar on a 757), so, when it captures the altitude (meaning that elevator pitch commands will now be to maintain the altitude instead of a climb speed) in VNAV, the airplane FMS commands the autothrottles to maintain that speed.
As the airplane is leveling off, and the throttles are coming back, because it was at full power, and because it was climbing so rapidly, and because the throttles are coming back smoothly, the airplane frequently gets well ABOVE V2+15, so those throttles just keep coming smoothly back to idle to get the airplane to the commanded V2+15 speed. You get a pretty big change in pitch and power.
There's a somatogravic illusion that the airplane is falling, or decelerating...it is decelerating, but just a bit. Your inner ear is lying to you: it went from accelerated motion (takeoff, and then the climb) to steady motion, but the change feels to you like you're decelerating and now descending...you're not, but it feels that way.
Up front, it feels rough - a too-agressive correction to maintain altitude and a glitch in wanting to maintain an inappropriate speed. When the airplane levels off, we want it to keep accelerating through the flap retraction speeds, but if VNAV captures at low altitude, it wants to stay at V2+15.
So...
Experienced 757 drivers use FLCH when leveling at low altitude to prevent the glitch above. When FLCH captures the altitude, we simply dial up a manual speed selection and the power remains steady as the airplane levels off and begins to accelerate.
Pilots that come from other airplanes are used to the more sophisticated VNAV, and they tend to prefer the more automatic mode as "better" but in this case, VNAV just isn't a good choice.
In fact, our flight manual cautions that "Flight Level Change should be used for level-offs of 3,000 feet and below" - which is nearly every day in EWR...
So, full power takeoff, and a crew that elected to use VNAV - and you got a fairly uncomfortable level - off.
It's a 757 thing...
Cheers,
Astro
