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Landing Approach Speed - Tom Clements, King Air Academy


Landing Approach Speed - Tom Clements, King Air Academy

In the POH's Landing Distance charts, there is a table that lists "Approach Speed" as a function of landing weight. The heavier the weight, the faster the speed. Since the landing comes at the end of the approach, some pilots, understandably, are unsure as to exactly what this speed is and when it applies. Some believe that the entire approach, perhaps from the outer marker inbound, should be flown at this speed. No, not usually.

The use of the term VREF seems quite common nowadays. Although originating in the certification rules for Transport Category airplanes, it has been adopted by both manufacturers and operators of light planes. VREF, Reference Speed, is simply 30 percent above stall speed, or 1.3 x Vs. That is exactly what Beech's Approach Speed is, nothing more or less than 1.3 x Vs. So, in this context, Approach Speed and VREF are identical, one and the same.

Lately, there has been a bit of a flap - pardon the pun - in some training organizations coming from the move to prohibit or forbid landings in which Approach flaps are used, not full flaps or no flaps. You see, the argument goes, Beech publishes data for full and no flap landings but nothing for approach flaps, so it must be a no-no.

Really? Really?! Folks, I ran the Beechcraft Factory Training Center in Wichita back in the 1970s and we advocated and practiced plenty of approach flap landings. Specifically, this was the preferred configuration for single-engine landings and for low ceiling and visibility ILSs in which the runway did not come into view until under 500 feet above touchdown. Since the POH did not present landing distances based on approach flaps, we calculated the no-flap distance and merely made the logical assumption that the approach flap landing distance would not be longer, since the applicable VREF speeds were lower. 

Of course, there are exceptions to every rule and if the runway for the single-engine or low ILS landing was not long enough to accommodate the choice of approach flaps, then we'd need to find a different runway or go ahead and use full flaps, even though it was not the preferred choice.

And why is an approach flap landing preferred in these two cases?

In the case of the low ILS, selecting full flaps close to the runway does two undesirable things. First, it destabilizes the approach, giving a ballooning tendency and a need for trim and sight picture changes. Second, it exposes the airplane to a split flap situation uncomfortably close to touchdown, with little time to recognize and respond to the situation. 

In the case of the one-engine-inoperative landing, Beech makes it clear that the selection of full flaps cancels the go-around option. Granted, why would we ever choose to make a single-engine go-around? With proper runway and weather selections, the need for that maneuver should be exceedingly low. But, on the other hand, when the Airbus we'd been following blew a couple of tires, stopping on and blocking the runway, wouldn't it be nice to do an uneventful go-around and either use another runway or even another airport? However, if we'd already gone to full flaps, then we'd better land in the grass or on a taxiway beside the blocked runway.

Let's get back to our discussion of the landing distance chart's Approach Speed table. In the Associated Conditions presented on the chart, a sink rate or descent angle is specified at the threshold crossing height, the standard fifty feet Height Above Touchdown (HAT). The combination of configuration, power, speed, sink rate, height above the threshold, and braking and/or propeller reversing activity are the variables that determine the charted landing distance. If we are remiss in "nailing" any of these, then the distance will be off by some amount. And isn't it somewhat laughable that "Maximum Braking" is always specified?! Unless your flight department has an unlimited tire and brake replacement fund, I'll wager few of us have ever, ever, used "Maximum Braking."

So Approach Speed or VREF is the speed at fifty feet above touchdown. How we establish that speed is usually different from pilot to pilot. A Bob Hoover could probably do an engine-out four-point roll on final and still nail VREF perfectly at the appropriate point. As for me, I'm going to need a little extra time to get established and stabilized. That's why I'll select landing flaps no later than 500 feet HAT and then bleed off speed and re-trim so as to be stable at VREF at the threshold. Thus, my actual speed on final approach to landing is above VREF until the very end, the last 100 feet or so. We turboprop pilots have some flexibility that the jet-jots lack. Their need to be fully configured and stabilized for at least the last 500 feet does not apply with our thrust-responsive and drag-responsive propellers.

Although difficult for some to accept and do, one of the basics of the landing distance testing is that power is at idle from the fifty-foot threshold point to touchdown. Given that fact and the need for a flare before touchdown, the actual touchdown point is close to 1,000 feet past the fifty-foot height mark and the touchdown speed is typically ten to fifteen knots below VREF. Since the touchdown speed is virtually impossible to accurately control, it is never posted in the POH.

Although various rules-of-thumb exist for how much VREF should be adjusted when using approach flaps for landing, how can we find the exact number to use for our various landing weights? We do it exactly the same way that the Beech flight test engineers determined the flaps 0% and flaps 100% VREF speeds. It may take an hour or so, maximum, but it is a worthwhile exercise. (Unless your training provider already has done the work and published the speeds for you.)

Here's what you do: First, go to the stall speed chart in the POH and determine approach flap VS for the same weights used for full and no flaps. Notice that the speeds here are presented as Calibrated Airspeeds, CAS. Second, multiply the speeds you have found by 1.3. Third, now go to the Airspeed Calibration chart in the POH and convert the CAS values you have calculated to IAS values, making sure you use the line for Approach flaps. Round your answers to the nearest knot. Bingo! Now you have the exact VREFs to use for the rather-rare approach flap landing.

Author: Tom Clements, King Air Academy

Image: @theplanesnapper (IG)