Approach Speed and Mass

[GroundBound]

I came across this description of approach speed considerations in documentation intended for budding ATCOs.

The ground speed on final approach will be affected by the following factors:

Aircraft mass -
As aircraft mass increases, more lift is required to balance the weight. The minimum speed at which the required lift can be achieved will be higher when the aircraft has a greater mass (already explained with the flight envelope). Thus the airspeed on final approach depends on aircraft mass, that is, an aircraft is expected to fly with higher airspeed on final approach when its mass is greater (minimum speed increases with aircraft mass).

This could be understood as a heavier B747 will have a higher approach speed than a lighter B747.

Does the approach speed within a given aircraft type differ depending on its weight/mass? I wouldn't have thought so, but then I'm not qulified on such types.

I can understand the stall speed will increase with weight, but I thought the approach speed would have sufficient margin above the stall speed that the weight/mass differences would not have any significant effect.

Any clarifications would be greatly received?

GB

[Gufo]

Does the approach speed within a given aircraft type differ depending on its weight/mass?

Yes, it does. At least, if we're talking FAR-25 aircrafts, which I think was the case.

Within the same type, approach speed varies upon mass and flap setting. On a medium size machine (i.e. A320, 737) such speeds may range -very roughly- from 125 to 150 knots IAS. This ain't including wind corrections or abnormal conditions concerning hydraulics and/or flaps and slats.

Cheers :)

[Tinstaafl]

Yes, it varies.

Landing speed (more accurately, Vat or Vref, the target speed over the threshold) is predicated on some multiplier over & above stall speed in that configuration, typically 1.3 x Vs. Approach speeds in turn will be some buffer above Vat/Vat)

[Intruder]

This could be understood as a heavier B747 will have a higher approach speed than a lighter B747.
Indeed! As low as 118 knots (or less) at minimum landing weight to as high as 157 knots at max landing weight (Vref30 + 5 knots -- normal landing)!

[john_tullamarine]

As a minor variation, some light aircraft have been certificated by running performance data for gross weight only - saves a few dollars in a small budget exercise. In such cases you would see a constant approach speed scheduled and, on occasion, longer distances for lighter weights.

[GroundBound]

Thanks Guys,

from your responses, I presume you must determine the Landing Weight and then determine the corresponding approach speed.

This would mean that every approach will potentially be at a different speed, depending on current weight?

And depending on type, approach speeds may vary by as much as 30kts?

I guess I've learnt something, then. :ok:

[Intruder]

This would mean that every approach will potentially be at a different speed, depending on current weight?
Yes.

Add in different aircraft types and configurations (e.g., flap or other hydraulic problems), and the difference in airspeeds can approach 80 knots or more.