getting to grips with aircraft performance 如何掌握飞机性能 2(4)

Actual headwind is measured by ADIRS, and the tower wind is entered on the MCDU.
1 When the auto-thrust is used or to compensate for ice accretion on the wings

3.1.3. Reference Speed: VREF
In case of failure in flight, emergency or abnormal configuration, performance computations are based on a reference configuration and on a reference speed. VREF means the steady landing approach speed at the 50 feet point for a defined landing configuration. For Airbus, this configuration is CONF FULL.
That gives:
 VREF= VLS in CONF FULL
In case of a system failure affecting landing performance, Airbus operational documentation indicates the correction to be applied to VREF to take into account the failure:
VAPP = VREF + .
VINOP
Another speed increment can be added to VAPP to account for wind, when needed.
3.2. Actual Landing Distance (ALD)

3.2.1. Manual Landing
“JAR/FAR 25.125
(a)The horizontal distance necessary to land and to come to a complete stop from a point 50 ft above the landing surface must be determined (for standard temperatures, at each weight, altitude and wind within the operational limits established by the applicant for the aeroplane) as follows:
.  
The aeroplane must be in the landing configuration

.  
A stabilized approach, with a calibrated airspeed of VLS must be maintained down to the 50 ft.”

During airplane certification, the actual landing distance is demonstrated as follows:
It is the distance measured between a point 50 feet above the runwaythreshold, and the point where the aircraft comes to a complete stop.
To determine this actual landing distance, several conditions must be achieved:
.
 Standard temperature

.
 Landing configuration

.
 Stabilized approach at VLS (or VMCL whichever is greater) for the configuration for manual landing.

.
 Non excessive vertical acceleration

.
 Determination on a level, smooth, dry, hard-surfaced runway

.
 Acceptable pressures on the wheel braking systems

.
 Braking Means other than wheel brakes: Spoilers, reversers (except on dry runway), can be used when they are safe and reliable.

Actual landing distance is also certified with degraded braking means (spoiler inoperative, one brake inoperative…).

Actual Landing Distances are certified on dry runways for all Airbus aircraft, certified on contaminated and icy runways for all fly-by-wire aircraft and published (for information) for wet.
Demonstrated landing distances will not account for reversers on dry runways. The reverse thrust influence may be considered on contaminated runways.
On dry runways, landing distances are demonstrated with standard temperatures, according to JAR/FAR 25. However, on contaminated runways, Airbus decided to take into account the influence of temperature on landing distance demonstration. This choice ensures added safety as it gives a conservative ALD.
　
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