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Touchdown point limits

Quantifying when a little float is too much

The Citation Jet Pilots Safety and Education Foundation (CJP SEF) “Safe to Land” initiative (see “Owning the Go-Around Button,” May 2022 AOPA Pilot, Turbine Edition) is now live.

Illustrations by Charles Floyd
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Illustrations by Charles Floyd

There are two critical components to this initiative that attempts to reduce the frequency of landing overruns: delivering the aircraft to the runway in an appropriate energy state (not too high or fast across the threshold), and ensuring the rollout and stop can actually be performed with the pavement present. The revised stabilized approach criteria and verbal callouts previously discussed address the first issue by creating a more realistic stabilized approach policy, with the hopes of greater pilot acceptance and regular utilization.

However, even an approach perfectly flown at VREF delivering the aircraft precisely 50 feet over the threshold can then become destabilized over the runway, particularly in gusty wind conditions. Furthermore, physics can’t be cheated, and in the case of non-dry runways a seemingly adequate runway length has too frequently been found insufficient by crews experiencing an excursion. Enter the touchdown point limit (TPL).

Just as the improved stabilized approach criteria separate targets (e.g., the pilot should try to be no more than 10 knots over VREF at 500 feet above field level) from limits (if still more than 10 knots fast at 200 feet a go-around should be initiated), the TPL identifies a limit of how far down the runway the pilot will accept touchdown. The target remains the 1,000-foot aiming point markings, but now a pilot is presented with a concrete point for every landing at which the mains must be on the runway for the landing to continue.



Using airport diagrams and/or satellite imagery such as Google Earth, a pilot can choose an easily identifiable reference, such as a taxiway or runway intersection, to mak the TPL. In the example of Runway 34 at HPN, a pilot can utilize ForeFlight's measuring tool to identify the left turn onto taxiway Kilo as being just over 1,700 feet from the threshold, or a slightly conservative visual TPL reference, given the actual TPL of 1,813 feet.
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Using airport diagrams and/or satellite imagery such as Google Earth, a pilot can choose an easily identifiable reference, such as a taxiway or runway intersection, to mak the TPL. In the example of Runway 34 at HPN, a pilot can utilize ForeFlight's measuring tool to identify the left turn onto taxiway Kilo as being just over 1,700 feet from the threshold, or a slightly conservative visual TPL reference, given the actual TPL of 1,813 feet.

Calculation of the TPL appears complex at first glance, but with proper tools it is simple and quick in practice: The factored landing distance is subtracted from the landing distance available, then 1,000 feet is added. Breaking down the steps helps understand the logic.

Manufacturers’ published landing distances (unfactored distances) represent the best of the best landings. During certification, test pilots collect data on how much runway is needed to stop an aircraft that is flown perfectly to the threshold at VREF and 50-foot height. At the threshold, power is brought to idle, the airplane touches down on or shortly before the aiming point, and within one second of main wheel touchdown full braking is applied.

But even very good pilots will experience small deviations from VREF and/or threshold crossing height on any given approach, and are not likely to immediately apply full brake force.

Because of these variations from perfect technique, the FAA recommends the application of a landing safety factor to all turbojet operations. While a minimum of 15 percent’s worth of additional runway is recommended, the CJP SEF has elected to use a more conservative, 25 percent factor. Thus, if a landing is calculated to require 3,000 feet, the pilot would treat 3,750 feet as the minimum acceptable runway—the factored landing distance.

When a runway is in a non-dry state it is critical to enter the proper runway conditions during performance calculations. In 2015 the FAA published Safety Alert for Operators (SAFO) 15009, warning operators that analysis of multiple landing excursions revealed “the braking coefficient of friction in each case was significantly lower than expected for a wet runway...on both grooved and ungrooved runways.” A committee was established to investigate the issue, and in 2019 SAFO 19003 was published with the investigation’s results.

SAFO 19003 provides easy-to-follow guidelines for utilization of a manufacturer’s non-dry performance information. It was found that even on a grooved runway, if heavy rainfall conditions exist, stopping performance will be more accurately reflected in the contaminated (standing water) performance numbers, not wet numbers. If the runway is not grooved, the friction will fall off more quickly; the shift from wet to contaminated performance data should occur with moderate rainfall.

The difference in required runway can be substantial. An Embraer Phenom 300 landing at mid-weight will only require 2,887 feet (unfactored) to stop on a wet runway. Should heavy rain be present (on any surface type), or moderate rain be falling on an ungrooved runway, the unfactored distance jumps to 4,427 feet using the contaminated data. Adding a 25 percent safety factor, more than 5,500 feet of runway would be needed for reasonable safety margins to exist.

Putting the pieces together, a pilot expecting rain at time of landing can calculate the TPL for both wet and contaminated conditions, noting that in some cases safe landing will be impossible on a shorter runway if the conditions transition to contaminated.

On the facing page we see the results of a popular runway analysis program’s calculation of the factored landing distance (“80 percent landing factor” is another way of stating a 25 percent margin was added) for a Citation M2 landing on Runway 34 at New York’s Westchester County Airport (HPN) in heavy rain. Given the rain intensity, the pilot has entered runway conditions of “.125 water,” which are the most conservative standing water values for the M2. The factored landing distance of 5,736 feet is displayed conveniently next to the landing distance available of 6,549 feet. We can easily see that the pilot will have 813 feet more runway than needed for stopping.

As the performance is predicated on the pilot touching down on the aiming point markings, adding the assumed 1,000 feet of “air distance” to the “surplus” runway of 813 feet gives us our TPL: 1,813 feet. The calculation has shown that if the wheels are on the ground just at the TPL, there should be just enough pavement left in front of the airplane to stop. Should the airplane float past the TPL, stopping on the runway is in question, and a go-around would be warranted from the float.

Identifying the TPL numerically is of little use to the pilot without correlating an appropriate visual reference on the surface. Using airport diagrams and/or satellite imagery such as Google Earth, a pilot can choose an easily identifiable reference, such as a taxiway or runway intersection, to mark the TPL. In the example of Runway 34 at HPN, a pilot can utilize ForeFlight’s measuring tool to identify the left turn onto taxiway Kilo as being just more than 1,700 feet from the threshold, a slightly conservative visual TPL reference given the actual TPL of 1,813 feet.

Finally, an important limit of the TPL methodology must be understood. The math behind the TPL calculation is predicated on the airplane being at or near VREF crossing the threshold. If an aircraft is flown across the threshold well above VREF the TPL might be “made” by forcing the airplane onto the runway, yet stopping on the pavement might not be possible. TPL calculations complement, but in no way replace, a properly flown stable approach and landing. Neil Singer is a corporate pilot, designated examiner, and instructor in Embraer Phenoms and Cessna Citations. He has more than 10,000 hours of flight time.


Neil Singer
Neil Singer is a corporate pilot, designated examiner, and instructor in Embraer Phenoms and Cessna Citations. He has more than 10,000 hours of flight time with more than 20 years of experience as an active instructor.

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