We might have misled you, or perhaps the combination of our statements here is confusing. I know it is to me.
Your understanding that V1
is "the minimum speed for the plane to lift off on failure . . ." is not correct. There is another speed, VMU
or velocity-minimum unstick which is the lowest speed at which (under the given conditions) the plane can be pulled off the ground - with and without an engine failed. These demonstrations are done by the factory test pilots for the FAA and make for some very dramatic video.
No sir, V1
is the takeoff decision speed. The other descriptions of it are valid but functionally it is a speed we will reach during the takeoff roll before we rotate and lift off. "Decision speed
" is a little misleading because the decision was already made before we start rolling.
The decision is: If we have a problem before reaching V1
we are going to reject the takeoff and keep our problem on the ground. If we have a problem after
we are going to continue the takeoff. And we are going to go flying no matter what the problem is because taking the plane around the traffic pattern for a more normal landing is less risky than attempting to stop it. For one thing we might not
have sufficient runway/stopway to stop the plane without damage after V1
Now, how do we get these speeds. Well they come off a table, often bound in a little spiral book, otherwise a large chart we can pull out of a pocket on the plane. We follow the instructions (we know them by heart after a few uses) and we get the proper speeds. OR
: The come up on ACARS or a hard copy could be handed us when the door is closed. This version has correct speeds (and some other data) for this
takeoff: Our actual weight/balance, the runway in use (its length and slope) the actual temperature and wind corrections as needed. Now these speeds might come from the same kind of chart we have in the cockpit but likely the computer generated them using the same algorithms that were used to build my hand-held V-speed chart, as supplied by the airplane's builder.
While looking for something else last night I ran across a V-Speed chart for Boeing 737 that is still in use at an airline where I used to teach (among other things) B-737 performance. Let's find one actual set of numbers off that chart for one specific condition:
On this chart, for each takeoff you would begin with the engine thrust rating to be used, 20,000 lbs for one side of the chart and 22,000 for the other. Conditions permitting, they may operate their 22K engines as 20K in a derated takeoff. So, we are going to use 22K or 22,000 lbs of thrust.
On the 22K side there is a small table I enter on the left side at the airport elevation. I go across to a point above the current (or assumed - in case of a reduced power takeoff) and since we are doing that I will go across to a point above 30o
C and what I have is a graph of shaded and non-shaded areas, each marked by a letter. I find that this elevation and temperature puts me in area "C"
Next I make my flap selection. It offers me takeoff at flaps 5 or 15. We are going to use flaps 5 today. So I enter the Flaps 5 table at the top under column "C" which I got in the above paragraph. Column C will show me a column of V1
Next I enter the same chart at the left side at my actual takeoff gross weight. Let's say we are at 135,000 lbs. On the 135K line I go across until I am in column "C" This box tells me that my speeds are to be:
Now there are two more small tables on this page. One gives me a reduction to V1
for a 2% down slope of the runway I may increase V1
by 3 knots if it slopes UP
. This is because I can stop easier going uphill. I can delay the "decision" a little bit for this. It also says that I must reduce V1
by 3 knots if I have a 15 knot tailwind (longer stopping distance) or increase it 1 knot if I have a 40 knot
headwind. Not likely to be using either of those.
The last table deals with VMCG
which is only going to be a problem if my actual takeoff gross weight is less than 119,000 lbs. The inputs on this table are Actual outside air temperature and pressure altitude.
So, here are the numbers again.
And it will be these that we "bug" on the airspeed indicators and the non-flying pilot will call during the takeoff.
Hope that helps rather than just muddying the water further.
Happiness is not seeing another trite Ste. Maarten photo all week long.