When it comes to performance planning in jets, however, the saying can be flipped on its head: Takeoffs are mandatory, but every particular landing should be thought of as optional. By this, I mean that once an airplane finds itself at a given airport, at some point it is going to have to depart that airport. Performance planning for takeoff becomes an exercise in reverse engineering: Given the unalterable conditions of runway, altitude, and terrain—and the expectations of temperature, wind, and precipitation and/or icing—what kind of loading can we permit and successfully depart the airport with an out during every stage of the takeoff and departure?
For landing, however, pilots can find themselves in a different situation: They can no longer alter the aircraft’s weight or change the environmental or airport conditions. Thus, the performance calculations are executed to a binary outcome: Landing can be safely effected with adequate margins, or it can’t—and a diversion to a more suitable airport must be performed.
While takeoff performance planning is largely driven by mitigation strategies for a theoretical engine failure during each phase of departure, landing performance planning is mostly a question of energy—kinetic energy, to be specific. Given the energy present in a landing airplane because of its mass and velocity, how much runway will be needed for the brake system to absorb that energy and stop the airplane? An increase in mass (landing weight) or velocity (groundspeed at touchdown), or a decrease in braking effectiveness (runway surface friction decreased because of moisture) means that more runway will be necessary.
Unfortunately, any increase in landing weight is compounded by a necessary increase in landing speed, because of the calculation of our first landing speed, VREF. Calculated for every landing, and based on the landing flap position (when options exist), icing conditions, and landing weight, VREF is the approach airspeed at which aircraft should cross the runway threshold. It represents 130 percent of stall speed in the landing configuration and at the landing weight, so as landing weight increases, so does stall speed and VREF.
Before we can begin the complex calculations that tell if the aircraft weight and VREF will allow for a safe landing on the runway, a more basic restriction on landing weight must be complied with—one that relates to the other two landing speeds typically calculated. These are the approach climb speed and final takeoff speed (used similarly in this case for landing).
Go-arounds, flaps, and approach climb speeds
Approach climb speed, called VAPP or VAC by two large manufacturers of light jets, is selected to give the best possible climb performance during a go-around with one engine inoperative. With maximum takeoff power (time limited) on the operating engine, the landing gear retracted, and the flaps in the go-around position, the aircraft must be capable of achieving a 2.1 percent climb gradient: climb up 2.1 feet for every 100 feet of forward flight.
While the manufacturer has latitude in selecting this approach climb speed, it cannot be less than 1.5 times stall speed in the go-around flap configuration, and the flap setting selected for go-around cannot raise stall speed more than 10 percent from stall speed with the flaps in the landing position. So, while retracting flaps fully for the go-around would result in a much better climb gradient, it is not allowed as the stall speed increase would be too large. Rather, a partial flap retraction is performed, and this partial flap setting is maintained for the initial part of the go-around.
During conditions of high altitude and temperature, the ability of the aircraft to achieve the mandated 2.1 percent gradient can be an issue. For the smallest light jets that have a lower thrust-to-weight ratio than their larger and higher-performing siblings, this inability to meet the gradient at high weight can cause an approach climb limit on landing weight. Some manufacturers can mitigate this limit to landing weight by allowing for a reduced flap landing setting—trading off a higher VREF and longer landing distance for the less restrictive approach climb limit present with the lower flap extension during a go-around.
Consider an Embraer Phenom 100 landing during summer at Colorado Springs (COS), with an outside temperature of 30 degrees C. If the landing is performed with full flaps (36 degrees of extension), the go-around must be performed with a 26-degree setting. This setting carries enough drag that the aircraft would be approach climb limited to a 7,600-pound landing weight. That’s an impractically low limit if any passengers are aboard.
With a landing conducted with the lower 26-degree setting, however, the go-around is flown with only 10 percent of flaps. This creates significantly less drag during the climb-out, and so the weight limit jumps to more than 9,500 pounds—nearly the structural landing limit. For this reduced flap setting an extra five knots of approach speed is needed, which translates to an extra 300 feet of runway requirement. That’s a fair tradeoff for the increased landing weight.
After go-around, once the aircraft has reached a safe altitude above the ground, the aircraft is accelerated to the final climb-out speed (VFTO), flaps are retracted, and thrust reduced to the maximum continuous setting. This “en route” climb must be capable of maintaining a 1.2-percent gradient with one engine failed, but is less commonly a restriction on landing weight than the approach climb. When landing at low runway elevations and high temperatures, however, it can be restricting, and must be observed just as the approach climb limit. Manufacturers typically combine the approach and en route climb limit information onto one chart that displays the more restrictive limit for a given altitude and temperature.
Engine-out terrain clearance
Of interest is the absence of a requirement to demonstrate that an engine-out missed approach path can clear terrain. Even for-hire operations are not required to perform an obstacle clearance analysis for one-engine-inoperative missed approaches or rejected landings. Instead, the FAA recommends that “it is appropriate to provide information to the flight crews on the safest way to perform such a maneuver should it be required.”
Because a missed approach is started both farther and higher from the runway end than a takeoff, using takeoff engine-out climb-out data (in what’s known as runway analysis) is considered a safe option to ensure that engine-out go-around performance is adequate. Fortunately, engine failures are rare in jet aircraft, as are missed approaches; the confluence of both is a remote occurrence.
Unfactored versus factored landing distance
It’s much more likely that a landing is attempted, and a go-around isn’t needed. Pilots need to understand that the unfactored landing distance requirements they calculate from manufacturer-supplied data are predicated on the aircraft crossing the threshold at exactly VREF. Any speed over VREF will increase landing distance above the book value. Because energy increases with the square of velocity, an aircraft with a VREF of 100 knots crossing the threshold at 110 knots has 10 percent more speed, but 21 percent more kinetic energy. So, an aircraft expecting a 2,000-foot ground roll from book values would need an extra 420 feet if VREF is exceeded by 10 knots.
During conditions of high altitude and temperature, the ability of the aircraft to achieve the mandated 2.1 percent gradient can be an issue.Unfortunately, the accident history shows that when one landing parameter is exceeded, many others often are as well, and all contribute to a longer landing roll. The unfactored landing distance is validated by a test pilot crossing the threshold at exactly 50 feet, bringing the power to idle, descending at up to 480 fpm to touchdown, and initiating full braking one second after the main wheels touch down. Any variation from this aggressive profile will increase the landing distance. An extra 10 feet high above the expected 50 feet will add 200 feet to the landing distance, and any floating or delay in braking will add 170 feet per second when the aircraft has a typical touchdown speed of 100 knots.
Adding together the possible variations from ideal technique, we see that an expected 3,000-foot-long landing can easily use 4,000 feet or more if the pilot isn’t on top of all the variables. Because of this, the unfactored landing distance should never be thought to be an adequate amount of runway. The FAA states that “The unfactored landing distances in the manufacturer-supplied AFM [Airplane Flight Manual] reflect performance in a flight test environment that is not representative of normal flight operations.”
For-hire operations are required to apply a safety factor to their landing performance planning, so as to have an extra two-thirds of the calculated runway required present as a safety margin. Thus, if the calculated performance says a landing in 3,000 feet is possible, the actual landing distance available can’t be less than 5,000 feet. While this factored landing distance calculation is not required for private operations, it is a widely accepted best practice to calculate and observe it.
Wet and contaminated runways
Utilization of a safety margin becomes especially important when landings are conducted on wet or contaminated runways. History has shown that jets depart non-dry runways at a rate 1,300 percent greater than from dry surfaces. Whether the runway is wet (defined in the wonderfully simple FAA term as “not dry”) or contaminated (when more than 3 millimeters of standing water, slush, or snow exists), performance planning rapidly becomes less precise and scientific.
Recent safety alerts issued by the FAA attest to the fact that “Several recent runway-landing incidents/accidents have raised concerns with wet runway stopping performance assumptions. Analysis of the stopping data from these incidents/accidents indicates the braking coefficient of friction in each case was significantly lower than expected.” A review of these accidents found that when landing on a runway without grooving—lateral channels that carry water off the runway surface—rainfall intensity more than light should signal that the runway has transitioned from wet to contaminated, while on a grooved runway heavy precipitation should do the same.
For one common light jet landing at a typical weight, the unfactored landing distance increases from 2,600 feet on a dry runway to 3,500 feet when wet, and to 4,600 feet if the water crosses the 3-millimeter threshold to a contaminated state. This jet that can normally land with an adequate safety factor on a 4,500-foot dry runway now needs nearly 8,000 feet of pavement for the same safety margin in moderate rainfall if the surface is not grooved. Few runways at smaller general aviation airports are this long, so it is unsurprising to see why runway overruns on wet and contaminated runway continue to plague light jets.
Pilots should calculate, before landing, whether they will have enough runway should conditions change from wet to contaminated. In some cases, the landing distance available will be sufficient for landing in any condition, while on shorter runways a landing may have an adequate safety margin only if the runway is dry or wet, but not if contaminated. In this case the pilot must be alert during the approach to any indication, whether from a runway condition code update, a pilot braking action report, or a change in observed precipitation rate, that the threshold to contaminated conditions has been crossed, and be prepared to divert to a preidentified alternate with a longer surface and/or better conditions.