There are dozens and dozens of target airspeeds and their abbreviations all start with the letter “V” for velocity. Every private pilot is required to learn and recite Vx for his airplane, which is the best angle of climb airspeed. And also Vy which is the best rate of climb airspeed. And probably Vso which is stalling airspeed in the landing configuration.
Va is also a biggie in any airplane because that is maneuvering airspeed, the maximum speed at which you should make full or abrupt control inputs. But one of the most important airspeed targets for pilots of jets or larger airplanes, Vref, hardly ever gets a mention in piston airplane flying. I think Vref deserves more attention.
Vref is the final landing approach airspeed target with the airplane configured for landing. The goal is to be stabilized at Vref airspeed and cross the runway threshold at Vref. It is the only airspeed that I can think of where pilots get no slack on the minus side during a checkride. Other target airspeed typically have a tolerance of plus or minus 10 knots, but for Vref, the rules are plus 10 knots is okay, but there is no tolerance for flying even one knot slower than the target.
Vref also comes with maneuvering restrictions. Typically you can slow to Vref only when bank angle is limited to 15 degrees or less.
Vref airspeed is determined during certification flying but is usually 1.3 times the landing configuration stall speed. Of course, stalling speed varies with aircraft weight, so at higher landing weights Vref is faster. Airport elevation, air density, runway length or other considerations on approach do not change Vref, only aircraft weight does. What Vref gives pilots is a 30 percent airspeed margin above the stall.
The concept of Vref is nearly as old as flying itself. Pilots quickly learned that if they slowed too much on approach the airplane could stall and hit short of the runway. Flying slowly in gusty conditions could also change an otherwise safe approach airspeed into an unexpected stall. But carrying too much airspeed on approach leads to floating and a long landing that could send you off the far end of the runway.
It was, however, the introduction of jets with their high drag and heavy weight on landing approach that enshrined Vref as the most important airspeed for all maneuvering in the airport area. Pilots who flew too slowly on approach in a jet could find themselves in a serious sinking spell. Jet engines take several seconds to spin up to full power so without a margin above stalling speed there was no way to arrest the sink. The 30 percent airspeed margin built into Vref is the cushion a pilot needs to recover from an unexpected sink.
In piston airplanes power response is quick, and the drag of flaps and leading edge devices is not as high as on a jet so Vref has not carried the same emphasis. Pilots of piston airplanes can get away with flying much slower than Vref even in windy and turbulent conditions—most of the time.
But why should piston airplane pilots give away the safety margin maintaining Vref provides? And maintaining Vref target on final brings predictability to every approach. If you always cross the runway threshold at Vref airspeed your touchdown point on the runway will be predictable every time.
To determine Vref airspeed for your approach you need to know what the airplane weighs. I mean what it weighs on approach, not what it weighed for takeoff, or what maximum certified weight is, but what the airplane weighs right now. In most piston singles the difference in stalling speed between the maximum and minimum landing weights is only a few knots, but remember, Vref is a very precise target to fly to.
In my Baron Vref varies from 96 knots at maximum certified weight, to 85 knots at the lowest weight I’m likely to land at. Many airplane flight manuals provide estimates of how much landing distance each knot above Vref adds but mine doesn’t. But, at 90 knots groundspeed, the airplane is covering 150 feet per second. If you float for five seconds 750 feet of runway is behind you so you can see that every knot matters.
In windy and turbulent conditions it often makes sense to land piston airplanes with less than full flaps, but most piston airplane handbooks provide no partial flap stall speed information. For my airplane I can look and see that at a typical landing weight flaps full down stall speed is 70 knots. Flaps up stall at the same weight is 79 knots. I could estimate that approach flap setting would yield a stall airspeed half way between the two numbers, but that would only be a guess. So I choose the flaps up stall to calculate a Vref of 103 knots. If it’s that windy runway length is shortened so the extra speed should not be a problem to get down and stopped.
Yes, you can fly your whole career in piston airplanes without ever formally calculating or flying a Vref airspeed. In piston airplanes close is usually good enough. But knowing, and flying, a true Vref brings precision to your approaches and landings as well as added safety. I like to have a plan and specific target for everything I do in an airplane.