Why Does VREF Vary with Altitude?

[Minuteman]

This is my spinoff of the TAS/CAS/IAS thread.

Why does VREF vary with altitude. This is a question that's bugged me for a while and I've never really found a good answer.

VREF is supposed to be "1.3*Vstall". An aircraft will stall in level, 1g flight at a given indicated airspeed, which is directly tied to a given dynamic pressure. However, I've encountered performance charts and tables that give different Reference Speeds as a function of (typically) gross weight, configuration, and altitude ... and I can't figure out why there would be a different limiting speed at different altitudes. An airspeed is a dynamic pressure, which is a part of a well known equation of lift, and shouldn't vary with altitude, right?

I know what you're thinking -- "compressibility", but typical landing speeds are slow enough that compressibility really shouldn't have an influence.

Here's kind of weird example where speeds at light weights get slower with altitude, and at heavier weights they get faster at higher altitudes:

(speeds in this table vary from Mach 0.17 to 0.26)

Any ideas? I'm mostly just curious and at a point where I'll believe pretty much anything ya tell me.

 

[ananoman]

That is the first time I have seen something like that. I only have experience with a business jets that weigh about half the example you give, but the ref speeds for the aircraft I am familiar with do not change with altitude. One is able to take off and land at pressure altitudes from -2000 to 13000 feet.

If I had to guess I would say it is probably caused by instrument error.

 

[ananoman]

Instrument error is usually greater at higher flap settings, how much do the ref speeds change if you land with less flaps?

 

[seagull]

Its a good question, and one that I asked many moon's ago also. The answer is not all that complex, actually, and you might notice that the V2 speeds have a similar movement.

The issue is that Vref, besides being an approach speed, is also the reference speed for V2 and the landing weight. In other words, you use Vref+5 to meet the same performance goals. Heavier weights likely require higher numbers at altitude because Vx is moving up on the chart, while lighter weights are able to gain more angle with a lower TAS.

Obviously, these issues will vary from aircraft to aircraft, but this should get you thinking in the right direction. Similarly, observe the effects of altitude on V1 and Vr, based on the limits placed on those speeds.

 

[SFLAX]

It has to do with density of the air. In less dense air you will need to fly at a higher AOA to obtain an given lift( Same Indicated airspeed). So at full gross and high density altitude you are already closer to stall AOA at an airspeed that in "normal" atmosphere will not result in such a high AOA. So as stall speed increases so will Vref. But I maybe wrong.

 

[seagull]

If you're at the same IAS, then, by definition, the Q-factor is the same, so the lift would be the same (excluding compressibility effects).

 

[Snow]

Yeah I flew into an airport with a 7700ft density altitude, the approach speed reads the same on the airspeed indicator but your hauling ass in regards to ground speed.

I looked in the performance charts, vref goes up one knot above 8000ft too, weird.

 

[Minuteman]

Thanks for the answers.

I think seagull was on the right track with the idea that the variation with altitude was a function of other speed limitations, rather than a direct VREF performance variation,

[ QUOTE ]
§ 25.125 Landing
VREF may not be less than:
(i) 1.23 VSR0;
(ii) VMCL established under §25.149(f); and
(iii) A speed that provides the maneuvering capability specified in §25.143(g)


[/ QUOTE ]

I can easily see a scenario where VMCL is the controlling limit here. At higher altitudes, an engine will produce less thrust at a go-around power setting, which means less asymmetric thrust, less yawing force to counter, and ultimately a slower airspeed.

That at least covers the lower half (lighter weights) of the table. In the Flaps 50 table (no guesses on the aircraft

), the situation is similar where the speeds decrease at low weights and increase at high weights, but to a lesser extent.