Originally Posted by
Timbo
That thing is a real kite if you are a little fast! I asked a Boeing engineer about that once when he was at our Drill Weekend talking about the KC 135. He told me they had to bump up the approach speeds on the 757 to give it enough rudder authority in the event of an engine failure/go around scenario. He said you could fly the 757 at bug minus 10 and you'd still have plenty of margin above stall...but not enough rudder if you had to go around and lost an engine at the same time. (at flaps 30 ref speed)
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This isn't a correct statement. Approach and stall speed is weight dependent. However, Vmca is not. The Vmca charts in the 757 AFM are only dependent upon temperature and pressure altitude. Which means that Vmca is really based on the amount of thrust produced. GA thrust is the same regardless of weight, so as long as the aircraft speed is above Vmca, there will be sufficient rudder authority. For our 757's, the Vmca is 104 or 107 kts(depending on RR or PW engines) at 80 F and sea level, which is near the max Vmca speed on the AFM chart. The Vref speed for the lowest weight published, 140000 lbs, is 109 knots(the minimum Vref speed). For any weight the approach speed exceeds Vmca, therefore a bump to the approach speed wouldn't be required, at least for rudder authority.