AINsight: Runway Excursions and Crosswind Takeoffs

 - March 26, 2021, 12:45 PM

The rate and number of runway excursions around the world have been holding steady for the last decade. To curb these accidents, industry groups such as the Flight Safety Foundation (FSF) are renewing efforts to tackle this threat to flight safety. Runway excursions account for nearly a quarter of all accidents for both large and small aircraft.  

Earlier this month, FSF and its international partners published the “Global Action Plan for the Prevention of Runway Excursions” (GAPPRE). This document is the foundation’s latest effort to reduce the number of accidents during takeoff and landing.

Based on years of research, GAPPRE offers recommendations to individual segments of the industry such as air navigation service providers, aircraft operators, manufacturers, and regulators. Understanding the complexities, interrelationships, and cumulative nature of the problem, GAPPRE is a coordinated effort to improve runway excursion risk and resilience management.

According to FSF president and CEO Dr. Hassan Shahidi, “Reducing runway excursions and continuing to improve the overall safety of the approach and landing phases of flight continue to be a primary area of focus for the foundation. We are gratified by the efforts of the many safety professionals who gave of their time and expertise to make the GAPPRE a reality.”

While GAPPRE is all-encompassing, I will focus on areas related to aircraft handling; specifically, runway excursions during takeoff with strong or gusty crosswinds. Next month’s blog will explore circumstances that affect runway excursions during landing with crosswinds. Breaking down runway excursions by phase of flight, roughly 20 percent occur during takeoff, while the remainder are in the landing phase.  

There have been some significant events involving runway excursions during takeoff. On Dec. 20, 2008, a Boeing 737-500 crashed during an attempted takeoff in Denver during a period of mountain wave conditions that produced extraordinarily strong surface winds. ATC cleared the 737 for takeoff from Runway 34R and advised the crew of reported winds of 270 degrees at 27 knots. Based on the reported winds, the crosswind component was calculated to be 27 knots—the AFM limit is 33 knots.

The accident report noted that during the takeoff roll, unexpected strong gusts up to 45 knots made maintaining directional control difficult. The aircraft departed the left side of the runway at 110 knots and traveled an additional 2,400 feet across moderately rough terrain—crossing a taxiway and service road—before coming to a stop with the fuselage broken in two pieces. Six of the 115 passengers were seriously injured, with another 41 sustaining minor injuries.

Performance calculations made during the investigation indicated that the aircraft rudder could produce enough aerodynamic force to offset the weathervaning tendencies created by the strongest winds encountered during the takeoff.

The investigation concluded that the strong winds encountered as the aircraft accelerated were more difficult than what the captain had experienced in prior line flights or simulator training. Moments before the excursion, the captain—because of stress—eased right rudder inputs, applied full right control wheel input, and used the steering tiller to regain control.

Those actions were ineffective and delayed the initiation of the rejected takeoff by three to four seconds. Furthermore, the investigation revealed that had the captain reapplied “significant right rudder,” the aircraft would not have departed the runway.

Another runway excursion event occurred on Jan. 2, 2015, in Stornoway, UK. This crash involved a Saab 340B attempting to take off from Runway 18. ATC provided a spot wind check of 270 degrees at 28 knots when the aircraft was cleared for takeoff. Recorded winds at the time of the accident varied from 261 to 291 degrees, with speeds ranging from 14 to 27 knots.

The Saab 340 does not have a takeoff crosswind limitation listed in the AFM, but the max demonstrated crosswind for landing is listed at 35 knots. Most operators use this as the limit for takeoff based on a return to the departure airport.

Investigators determined that during this initial part of the takeoff roll, the captain (the pilot flying) held the ailerons into the wind, maintained directional control with the nose wheel steering (NWS) tiller, and kept the rudders in the neutral position. At approximately 60 to 80 knots, directional control was lost when the captain removed his left hand from the NWS tiller. Flight data indicated the rudder remained in the neutral position and the right aileron remained into the wind.

The aircraft departed the runway surface at roughly 80 knots—the power levers were never reduced, nor was there a call to reject the takeoff—and traveled another 800 feet before coming to a stop. Once the aircraft departed the runway, it entered soft grass and crossed over a closed runway; there was substantial damage to the nose gear, both engines and propellers, and the underside of the aircraft. One passenger was seriously injured.

According to the accident report, “had the rudder been applied as directed in the operations manual, there would have been a reduced NWS requirement at any given speed and therefore there would have been a reduced likelihood of a changed directional effect when the NWS control (tiller) was released.”

The report also suggested that the rudder would have become aerodynamically effective at around 40 knots. It was also noted that the rudder never moved from the neutral position.

Inappropriate flight crew aircraft handling was a factor in each of the accidents described above. This factor has been identified in several other runway excursions during takeoff rolls in windy conditions. Much of this can be attributed directly to poor skills related to training in simulators that cannot realistically replicate low-level wind velocity. Likewise, there may also be an insufficient understanding of the basic theory of directional control or poor techniques (unapproved) used for control during acceleration to takeoff speed.  

A crosswind takeoff is a difficult maneuver. This maneuver requires a lot of “rudder and stick” skills (note: more rudder than stick). Directional control during the takeoff roll is maintained by the aerodynamic forces generated by the primary flight controls.

Regardless of aircraft size, configuration, or number of engines, the rudder, ailerons, and elevator are used to maintain control once there is a minimum amount of airflow to make each control surface effective. On most large aircraft, the elevator is used to maintain a downward force on the nosewheel to maximize its effectiveness up to around 80 knots.

Many of the skills learned during primary training are applicable here. Keep in mind, during the initial part of the takeoff roll (below 80 knots) directional control inputs are dependent on the aircraft systems. Options include NWS through tiller only, such as on the Saab 340, Hawker 700/800, and C-130 Hercules; NWS through tiller and rudder pedals, which includes most jet transports; or a non-steerable nosewheel such as those on Cirrus SR-series and older aircraft.

Regardless of the type, once the rudder becomes effective, the maneuver is the same. As the speed of the aircraft increases, each control surface becomes more effective, thus less input is required.

On aircraft equipped with roll spoilers, it is important to understand how much control wheel deflection can be applied without the spoilers extending. In most aircraft, it is around 10-degrees of aileron deflection. Too much spoiler deployment can exacerbate the tendency for the aircraft to turn into the wind.

Likewise, excessive roll control—causing roll spoiler extension—will compromise takeoff performance. As an example, V1 may be compromised by the reduced lift and increased drag potentially invalidating accelerate/stop distances and second-segment climb performance.

The FSF and its partners identified 35 recommendations for flight ops organizations in the GAPPRE publication; aircraft handling—improved training and higher fidelity simulators, among others—is just one of those recommendations.

From a global view, runway excursions continue to be a serious threat, GAPPRE charts a course for every organization involved in the safe movement of aircraft to mitigate the risk of a runway excursion. For pilots—as operators—the best way to mitigate the runway incursion risk is improved aircraft handling through training, great and timely briefings, and attention to detail when calculating takeoff performance data.

Pilot, safety expert, consultant, and aviation journalist Stuart “Kipp” Lau writes about flight safety and airmanship for AIN. He can be reached at stuart.lau3@gmail.com.