AM From Mexico, joined Oct 1999, 599 posts, RR: 2 Posted (10 years 5 months 2 days ago) and read 9419 times:
I've noticed that on Boeing aircraft, during the take-off preparations, at least on the 757/767 and the 777, V2 is dialed in the MCP speed window, and VNAV is simultaneously armed.
After rotation, the F/D pitch bar commands a target speed of V2+10 (or V2+XX). At some point during the initial climb (which I believe is thrust reduction altitude), the speed window goes blank, and speed is adjusted as planned according to thrust reduction altitude, acceleration altitude, initial climb speed, etc.
First of all, is this correct? And my question is, what is the purpose of selecting V2 on the speed window prior to departure? Why not V2+XX, or the initial climb speed after acceleration altitude (250KIAS, for example)?
Thanks in advance for your replies.
"... for there you have been and there you will long to return."
PhilSquares From , joined Dec 1969, posts, RR:
Reply 1, posted (10 years 5 months 1 day 23 hours ago) and read 9413 times:
On the 744, V2 is also selected. The reason is for an engine failure after V1. Should you lose an engine you'd want to fly V2+10 of if you're faster, you fly what you already have up to V2+20. In no case would you fly slower than V2. The speed dialed in gives you those parameters. So, in essence, the selected V2 is a minimum speed.
Once you start to clean up, then the FMS commands the various speeds during flap retraction and once clean will command V2+100 or 250 knots which ever is higher.
You select V2 because that's how the FMS is designed. Hope that helps.
DeltaGuy From , joined Dec 1969, posts, RR:
Reply 2, posted (10 years 5 months 1 day 20 hours ago) and read 9368 times:
You've got it right....on the 757/767, the same applies as you and Phil have both said. You want to have that V2+10 dialed in for the (hopefully unlikely) event that you'll lose an engine, or in Phil's case, more than one.
Woodreau From United States of America, joined Sep 2001, 1143 posts, RR: 6
Reply 3, posted (10 years 5 months 1 day 20 hours ago) and read 9373 times:
I think also V2 is speed which guarantees the minimum rate of climb required for obstacle clearance (200ft/nm) when you lose an engine.
Otherwise at airports where terrain climbs faster than 200ft/nm, a special single engine out obstacle departure procedure (e.g. Reno Nevada) is published to still allow the reduced rate of climb at V2 to get to a safe altitude.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.