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The simple ‘V’

Just don’t drop below VREF

By  J. Mac McClellan

Of the several “V speeds” we calculate in flying turbine airplanes, the simplest is VREF, the approach and landing airspeed. All you need to know is the airplane landing weight.

Illustration by Charles Floyd
Zoomed image
Illustration by Charles Floyd

Compare that to determining the V speeds for takeoff. For V1, VR, and V2 you need to know the takeoff weight, of course, but also, at minimum, air temperature and takeoff airport pressure altitude. You must also consider runway conditions such as standing water, slush, or snow. If those are present, you probably need to consult another chart far back in the airplane flight manual to find the correct takeoff airspeeds.

But to determine VREF only the landing weight matters. That’s because VREF is based solely on stall speed in the landing configuration. And VREF applies only in stable flight restricted to minor bank angles.

VREF is also unique in that it is the only operating airspeed that I can think of that has no margin of error when it comes to minimum-airspeed tolerance under the checkride standards. Other checkride airspeeds such as during steep turns or holding an ATC-assigned airspeed have a tolerance of plus or minus 10 knots. Allow the airspeed to wander below VREF on approach during a checkride and you’re going to flunk.

VREF is also the baseline airspeed we must observe during maneuvering during the entire arrival process. Each configuration will have a factor that must be added to VREF as we prepare to land. Typically, the minimum airspeed in the clean configuration, for example, will be VREF plus 20 knots. Or even more in some airplanes.

For most of us, VREF airspeed changes little from landing to landing because we usually fly within a routine weight range. But while the indicated airspeed may look familiar, the actual landing runway requirements can vary dramatically.If you need to maneuver, even in landing configuration, the minimum airspeed will be at least VREF plus 10 knots. That’s why we look up VREF and bug it on the airspeed indicator before entering terminal air space. We need to know what VREF is so we can add the “factors” to determine the minimum airspeed for all segments of the arrival.

Under the rules, VREF is the steady-state stalling airspeed in landing configuration multiplied by 1.3. In other words, a 30 percent margin above stall. That may seem like a big cushion, but in larger high-drag turbine airplanes that margin can disappear in a hurry in turbulence, wind shear, or improper power management by the pilot.

The apparent simplicity of VREF hides the actual complexity of calculating a safe landing distance. Because VREF addresses only indicated airspeed and its relationship to stall, it can’t tell us anything about landing distance. To know that we need to know our groundspeed, not indicated airspeed.

As you well know, true airspeed—thus no-wind groundspeed—increases over indicated airspeed as air density decreases. So, when air temperature goes up, or we are landing at higher elevations, our true airspeed, thus ground speed, will be higher than the VREF we see on the airspeed indicator. That means we touch down at a higher ground speed and need more runway to stop.

I’m sure you’ve noticed when landing on high-elevation runways that the approach looks different. Your airspeed is the usual, but the ground is whizzing by much faster. It’s vital to ignore that sensation and maintain VREF on the airspeed indicator.

To determine the landing runway length required after you look up VREF you’ll need to consider airport elevation (pressure altitude), wind, and runway slope. Runway contamination may also be a huge factor. A wet runway can double the stopping distance, and icy pavement can go off the charts.

For most of us, VREF airspeed changes little from landing to landing because we usually fly within a routine weight range. But while the indicated airspeed may look familiar, the actual landing runway requirements can vary dramatically.

VREF is simple, but it’s just the start of the calculation of required landing runway.

J. Mac McClellan is a corporate pilot with more than 12,000 hours, and a retired aviation magazine editor living in Grand Haven, Michigan.

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