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757 vert speed

Old 11th Mar 2010, 21:14
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757 vert speed

Hi

Anyone know why in V/S the thrust levers in SPD mode automatically come up an inch, even when above target speed. Not sure if any power is actually used but they certainly come off the stops.

Cheers
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Old 11th Mar 2010, 21:25
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Don't know the answer, but the autothrottle / autopilots on the 757 (and 767) does some weird things e.g. in VNAV descent below speed and above path, it will increase thrust to attain desired speed rather than diving down to regain the path and getting free speed correction. I am very surprised Boeing never came up with improved software for these things.
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Old 11th Mar 2010, 23:10
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Thats a very good question and I always wondered the same thing myself
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Old 12th Mar 2010, 02:33
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VNAV works great in climb on a 757. Descending VNAV brought up the power a lot so I didn't use it much. Found FL change more efficient with an idle descent all the way down. I usually flew over the VNAV descent point a few miles to not have to add power. I always considered the lower winds in the adjustments but rarely needed speed brakes because I didn't follow the VNAV path. I made a game of idle power from top of descent to spooling up at 1000 ft per opspecs and not needing spoilers or power. I didn't care what the FO did on his leg but it made my leg more challenging.
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Old 12th Mar 2010, 02:44
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The explanation I heard is that the Boeing designed the V/S mode to be used during the old dive and drive type Non Precision approaches, and wanted the engines to be spooled up at low altitudes, even though it only comes up very little, barely a spool-up.

It is always a better technique to use VNAV or even the FLCH, because the V/S can fly you away from an Altitude thus get you in trouble!

Last edited by UAL320Capt; 19th Mar 2010 at 02:33.
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Old 12th Mar 2010, 04:23
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Descending VNAV brought up the power a lot so I didn't use it much.
I wonder, would this be the concept of a "partial power descent"? Apparently, statistically speaking there are no significant fuel savings of an idle descent compared to a partial power descent, but the energy management is much better with partial power. Apparently, they offer longer engine life (less thermal shock) and better pressurisation control too.
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