getting to grips with aircraft performance 如何掌握飞机性能 1(19)

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V1 can be selected by the applicant, assuming that an engine failure has occurred at VEF. The time which is considered between the critical engine failure at VEF, and the pilot recognition at V1, is 1 second. Thus:

 

This speed is entered by the crew in the Multipurpose Control and Display Unit (MCDU) during flight preparation, and it is represented by a “1” on the speed scale of the Primary Flight Display (PFD) during takeoff acceleration (See Figure C3).

2.1.3. Rotation Speed: VR

VR is the speed at which the pilot initiates the rotation, at the appropriate rate of about 3° per second.
“JAR/FAR 25.107
(e) VR, in terms of calibrated air speed, […] may not be less than:
.  
V1,

.  
105% of VMCA

.  
The speed that allows reaching V2 before reaching a height of 35 ft above the take-off surface, or

.  
A speed that, if the aeroplane is rotated at its maximum practicable rate, will result in a [satisfactory] VLOF”

VR is entered in the MCDU by the crew during the flight preparation.

2.1.4.  Lift-off Speed: VLOF

“JAR/FAR 25.107
(f) VLOF is the calibrated airspeed at which the aeroplane first becomes airborne.”
Therefore, it is the speed at which the lift  overcomes the weight.
“JAR/FAR 25.107
(e) […] VLOF [must] not [be] less than 110% of VMU in the all-engines-operating condition and not less than 105% of VMU determined at the thrust-to-weight ratio corresponding to the one-engine-inoperative condition.”
The regulations consider the particular case of aircraft which are geometrically-limited, or limited by the elevator efficiency at high angle of attack.
An aircraft is said to be geometrically-limited, when, at its maximum angle of attack (the tail of the aircraft hits the ground while the main landing gear is still on ground), the maximum lift coefficient is not reached. In these conditions, the margins can be reduced, as follows:
“JAR 25.107 (only valid for JAR)
(e) […]  in the particular case that lift-off is limited by the geometry of the aeroplane, or by elevator power, the above margins may be reduced to 108% in the all-engines-operating case and 104% in the one-engine-inoperative condition.”

“AC 25-7A (only valid for FAR)
For airplanes that are geometry limited, the 110 percent of VMU required by §
25.107(e) may be reduced to an operationally acceptable value of 108 percent on the
basis that equivalent airworthiness is provided for the geometry-limited airplane.”

Airbus aircraft, as most commercial airplanes, are generally geometrically-limited. For those aircraft, certification rules differ between JAR and FAR, as summarized in Table C1:
JAR  FAR 
Geometric Limitation  VLOF ≥ 1.04 VMU (N-1) VLOF ≥ 1.08 VMU (N)  VLOF ≥ 1.05 VMU (N-1) VLOF ≥ 1.08 VMU (N) 
　
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