Ready for OSH: The Third Degree

Ready for OSH:  The Third Degree

Ready for OSH: The Third Degree

Precise speed control matters in achieving predictable touchdown point. No time is that more crucial than when landing at the at the world’s busiest GA airport in late July.

AirVenture is the “Holy Grail” pilgrimage nearly all pilots dream about. The controllers are beyond incredible as they safely manage thousands of operations, but they sometimes have high expectations. If the arrival does not go as planned, the go-around process is clearly spelled out in the NOTAM and includes specific details on what to do if the pilot is not comfortable. Note: the controllers handled approximately 22 departures in the 13-minutes prior to the event so “busy” seems hardly adequate.

On July 22, 2016, a Piper Malibu entered the right downwind for runway 27. The controller instructed the pilot to turn base “abeam the numbers” (actually the orange dot) and land on the green dot, about 2,500 feet down the runway. They cleared a twin Cessna to depart and instructed the pilot to “roll it around the corner and scoot!”

The controller amended the Malibu’s clearance and requested he land on the orange dot. It’s located about 1,500 feet “before the green dot” and actually coincides with the “previous” turn abeam point location. Try to visualize turning base abeam the numbers and then trying to land on the numbers. From the NTSB final report (CEN15FA311) “The pilot considered doing a go-around, but decided to continue the approach. He reported that about 250 to 300 feet above ground level, he pulled back on the power which resulted in the airplane entering a stall. He attempted to recover by adding full power, but the airplane impacted the runway in a right wing low, nose down attitude.” Three serious injuries.

If nothing else, flying is a dynamic environment and pilots pride themselves in their ability to adapt. Our willingness to bite off more than we can chew can cost us dearly, but it’s not uncommon to assume a controller would never ask us to do something beyond a normal operation. NEVER be afraid to say “UNABLE.”

Short-Field Landing & Stabilized Approaches

The young applicant was attempting to perform a short-field landing near the end of his commercial multi-engine check ride. We’d already completed the single-engine work and if he got this right, he’d pass the ride. His pattern was good as he turned base. Flaps set, hand on the throttles. Without warning, he pitched up and leveled off. The airspeed dropped 10-knots and as he crossed the threshold, he pulled both throttles to full idle and pushed the nose over hard. Hard also described the landing, and the PTS “at or within 100-feet” metric became irrelevant.

During the debrief, I offered him the opportunity to explain what happened. Seems he leveled off to ensure he “crossed the 50-foot obstacle” per the maneuver description. The check ride was over at that point, so I decided to review the differences between a “short field” landing and a stabilized approach.

A stabilized approach is one in which the pilot establishes and maintains a constant angle glide-path toward a predetermined point on the landing runway. An aircraft descending on final approach at a constant rate and airspeed travels in a straight line toward a spot on the ground ahead.

In a textbook approach, the aircraft crosses the runway threshold (the fence) at Vref on a 3-degree descent angle. If all goes well, you have about 8 seconds from the time you cross the threshold until touchdown. Just think how many landings it takes to get one minute of practice.

Vref is defined as 1.3 times the stall speed in the landing configuration adjusted for weight. You can fiddle with it a bit by adding half the gust factor. Most pilot operating handbooks, including Piper’s, only include landing distance information for the maximum landing weight. Since 1.3 V-stall will always occur at the same angle of attack, you can account for changes to Vref due to weight by using a conservative estimate of subtracting 1 knot for every 100 pounds below the maximum landing weight. As an added benefit, for every knot you subtract, you reduce your total landing distance between 100 and 150 feet. For a landing weight 400 pounds below gross, that’s between 400 and 600 feet of float. If you’re light and fast, the additional landing distance adds up quickly. More advanced aircraft compute the actual weight based on payload and fuel burn. It’s the only way you can achieve the published performance numbers.

Landing distances are based on a 3-degree approach at Vref and accept that the aircraft is not touching down on the first brick, rather at about 1,000 feet from the threshold.

Figure 1: In a textbook approach, the aircraft crosses the runway threshold (the fence) at Vref on a 3-degree descent angle.

As the Figure 1 shows, the aircraft will level off and “float” as it dissipates the remaining energy. The combination of speed and ground effect work together to balance the power reduction. If the reference airspeed (Vref) is correct, the float will be minimal. If your speed is high, you’ll float as it dissipates. If you’re slow, there won’t be any float and you’ll make that airplane-shaped smudge on the runway. Over-rotation is another concern. Excess speed allows the aircraft to climb (balloon) if you over-rotate. Pilots sometimes continue to raise the nose and at some point they may lose sight of the runway. If this occurs, a go-around/rejected landing is the only safe option. Excess speed also keeps us afloat, which increases our exposure in gusty/crosswind conditions.

My definition of an aiming point is the point on the runway where the nose wheel would hit if you forgot to flare. I use the beginning of the 500-foot fixed distance marks (double set of three), also known as the beginning of the touchdown zone. The smaller the point, the more accurate the landing. If there’s no 500-foot fixed distance markings, then I use the beginning of the second centerline stripe. Your selected aiming point is normally in-view as you transition to the landing attitude.

Runways with precision approaches usually have standard markings which include the threshold (beginning), touchdown zone markings (starts at 500 feet from the threshold), and an aiming point (starts 1,000 feet from the threshold).

Runways with precision approaches usually have standard markings, which include the threshold (beginning), touchdown zone markings (starts at 500 feet from the threshold), and an aiming point (starts 1,000 feet from the threshold).

As the aircraft crosses the fence, the pilot begins a “slow” power reduction such that the throttle reaches low/full idle just as the aircraft achieves the landing attitude just above the runway. It’s a bit problematic from the standpoint of a numeric reference, especially when you consider you shouldn’t be looking inside at this point anyway. Newer pilots occasionally “chop” the power (to idle) all at once. Not only does this reduce thrust, it also blocks airflow over the horizontal stabilizer/elevator which causes the nose to drop. Our new pilot pulls the nose back up, which causes a significant drop in airspeed and the subsequent, embarrassing impact. So much for stabilized. On the other hand, some hesitate to get the power back, and the additional thrust keeps the aircraft in ground effect for what seems like forever.

Getting it Right

Getting it right is tough and it’s a little like basketball: Someone can show you how to shoot a basket, but you’ve got to practice to start making points.

Where did the FAA come up with 50 feet? A little research uncovered something so simple it was almost embarrassing. A standard vertical light system (VASI/PAPI) is designed to have the aircraft at approximately 50 feet when it crosses the threshold. Pull out a precision instrument approach plate and look at the threshold crossing height (TCH). Most are close to 50 feet with small variances due to installation restrictions. I guess that means there’s no meaningful difference between a stabilized visual approach, a stabilized short-field approach and a precision instrument approach. If you’re on the glide slope/path, the vertical light system should present an on-path indication and you’ll also be on a 3-degree descent angle. A simple crosscheck: Dial up the precision approach if available and use vertical indication as a supplemental reference during visual approaches.

In the normal course of aviating, we get used to the tactile feel of the elevator, i.e., how much movement equates to how much pitch change. Pilots like to add a little speed because “it feels better.” Otherwise, as the speed decreases, the more we must pull back on the elevator to achieve the desired pitch change. The “feels better” pilot usually has poor trim habits and often lands with the trim near the takeoff position.

A popular practice during the final seconds of the landing is to simultaneously trim the nose up as power is reduced. This keeps the elevator forces light/closer to normal. The downside can be the unexpected pitch up forces during a rejected landing. It’s manageable as long as you are aware of the possibility. It’s important to remember that “trim” sets the speed of the aircraft; that is, the aircraft will seek out the speed it’s trimmed for if you release the controls.

Time to back up and clarify. The first figure shows the aiming point at the numbers. Figure 2 shows it 1,000 feet down the runway. Well, which is it?

Depends on what you need to do.

If you’re coming to Double Eagle II in Albuquerque, the precision approach runway is nearly 7,400 feet long. If you left the autopilot hooked up, the aircraft would crunch into the runway close to the 1,000 feet fixed distance markings, which is about the apex for the 3-degree descent. The large majority of twin and turbine use larger fields, and it’s easy to get complacent and tolerate less than precise speed control. After all, it’s only a few “extra” knots. Those who fly into challenging fields understand the importance of precise speed control and the value of angle-of-attack systems.

Landing at Elizabeth

Traffic pattern basics: Be at pattern altitude midfield, configure abeam the touchdown, begin the descent. At 45 degrees from the touchdown, turn base, configure and continue the descent. Plan the turn to final (~ 400 feet to 500 feet AGL) and stabilize the approach with the goal to cross the threshold at Vref about 50 feet AGL. The process allows for gradual reductions in altitude/speed. No power cuts or large pitch changes.

I instructed at a Malibu/Mirage Safety Foundation seminar in Groton, Connecticut a few years back and my Piper Mirage client for the session REALLY wanted to land at Elizabeth (0B8). Useable runway is about 2,328 feet. We had a long talk and decided to go for it, but only after he demonstrated he could touchdown accurately and on-speed. We completed nearly a dozen practice landings at Groton before giving Elizabeth a try. The first attempt ended in a go-around because of runway-width illusions, but the second was on target, touching down in the first hundred feet of the pavement. When you consider the descent time from 50 feet to touchdown, it only took about 100-seconds of practice.

I’m sure you wouldn’t want to “give away” the first 1,000 feet. In this case, your aim point would be before the runway, near the beginning of the displaced threshold.

I’m not suggesting readers go out and start calculating Vref and press for the minimums. I offer this as a plausible reason for why you might be missing the mark or floating down the runway. Like basketball, you need to practice in order to improve.

Always consider this: Trim directly affects speed and speed controls the quality of the landing – first – last – always. Power controls the location!

(A big thanks to Dan Sharpes, CFI/CFII for his data crunching assistance.) 

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