IFF/7000 From , joined Dec 1969, posts, RR: Reply 5, posted (12 years 9 months 2 weeks 6 days 1 hour ago) and read 3112 times:
It's a little more complicated;
The 3 most importand speeds on TO are v1, vr, v2.
1. V1: The highest speed during TO at which the crew has a choice between continuing TO or abort TO. So if you decide to stop when passed v1 you will not be able to stop the ac at the end of the RWY.
2. vr: The speed at which the pilot rotates in order to reach v2 at an altitude of 35 feet at latest after an engine failure.
3. v2: Is the speed that the ac has to maintain during initial climb after an engine failure.Minimum v2 is equal to 1.2x the stalling speed. For fly by wire ac 1.13x vs.
A lot of parameters are needed to compute these speeds. EX. weight, configuration, winds, RWY condition (wet,iced,snow), airport characteristics (elevation , temp,...),brake energy,tire speed,etc
Also when you have very long RWY's we use improved climb speeds, this means that because of the length of the RWY you can take a higher v1, vr, v2.
On A321 I've seen rotation speeds from 120Kts up to 165Kts.
Ambasaid From , joined Dec 1969, posts, RR: Reply 6, posted (12 years 9 months 2 weeks 6 days ago) and read 3106 times:
I thought that V1 had evolved into an action speed rather than a decision speed. Therefore you dont have a choice, once you reach V1 you are going.
I think that you will also find that the 1.13Vs value applies to aircraft certified after a certain date rather than fly by wire versus non-FBW, this change also incorporates the use of a 1g factor in the stall.
IFF/7000 From , joined Dec 1969, posts, RR: Reply 7, posted (12 years 9 months 2 weeks 5 days 23 hours ago) and read 3104 times:
I tried to keep it simple but you're right.
V1 : One common and misleading way to think of v1 is to say "v1 is the decision speed." This is misleading because v1 is not the point to begin making operational Go/No Go decision. The decision must have been made by the time the airplane reaches v1 or the pilot will not have initiated the RTO procedure at v1. Therefore, by definition, the airplane will be traveling at a speed higher than v1 when stopping action is initiated, and if the airplane is at a field length limit weight, an overrun is virtually assured. ( 1 sec is the min time that will exist between the engine failure and the first pilot stopping action).Boeing's Pilot guide to TO safety.
For a conventional ac, the ref. stall speed, VSmin, is based on a load factor that is less than 1g. This gives a stall speed that is lower than the stall speed at 1g. All operating speeds are expressed as functions of this speed (ex. vref = 1.3 VSmin). Because ac of the A320 family have a low-speed protection feature that the flight crew cannot override, the airworthiness authorities have reconsidered the def. of stall speed for these ac. All the operating speeds must be ref. to a speed that can be demonstrated by flight test. This speed is designated VS1g. Airworthiness authorities have agreed that a factor of 0.94 represents the relationship between VS1g A320 and VSmin for conventional ac.
v2 = 1.2x0.94 VS1g = 1.13 VS1g
Vref = 1.3x0.94 VS1g = 1.23 VS 1g
These speeds are identical to those that the conventional 94% rule would have defined for these ac. The A320 family have exactly the same maneuver margin that a conventional ac would have at its ref speeds. (Airbus FCOM 3.04.10)
IFF/7000 From , joined Dec 1969, posts, RR: Reply 8, posted (12 years 9 months 2 weeks 5 days 22 hours ago) and read 3097 times:
The only other fly by wire ac I know is the F16. But there you fly AOA instead of Vref. All the pilot need to do is set the chosen AOA on the HUD and fly down to the RWY.13° AOA gives good tail clearance, 16° AOA gives a very slow approach but a good chance to scrape the ventrals.
IFF/7000 From , joined Dec 1969, posts, RR: Reply 10, posted (12 years 9 months 2 weeks 5 days 21 hours ago) and read 3087 times:
Let me just add this. United Airlines Perfo:
Airbus certified the A320 with Optimized V-Speeds. It literally throws Balanced Field out the window. Basically, SFO Engineering has identified a ' typical' runway for each flap setting, and then optimize V1 to take advantage of the full length of that 'typical' runway.
Some airlines have made these calculations for Boeing also.
Ambasaid From , joined Dec 1969, posts, RR: Reply 11, posted (12 years 9 months 2 weeks 5 days 7 hours ago) and read 3061 times:
The VS1g applies to non-FBW aircraft as well and is used by Boeing.
The reason that i heard for its introduction was to level the playing field between the FBW and non-FBW aircraft, rather than having a test pilot declare the "stall", they got a bit more scientific about it and therefore removed the human error.
Airbus has always used Optimized V-speeds, we have them in the A300. Boeing also have the option, but not many operators use it.