Runway excursions have become one of the most common types of aircraft accidents worldwide. Most of them occur during landing with lateral excursions and are becoming a growing concern.
Typically, poor pilot technique or aircraft handling errors combined with environmental factors such as weather contribute to these events. Crosswinds in particular have proven to be a challenge. While these events are rarely deadly, lateral runway excursions, or veer-offs, often result in a hull loss.
Last month’s blog focused on runway excursions and crosswind takeoffs, specifically aircraft handling during periods of strong and/or gusty winds. As mentioned, aircraft handling and the environment are the two main contributors to runway excursions during landing. Other factors may include pilot decision-making ability (land/no-land decision) or understanding of aircraft performance limitations related to landing in a crosswind together with hazardous conditions such as a contaminated runway surface.
Pilots operate in a complex and very dynamic environment. Even the best laid plans, as an example, can be disrupted by changes in the weather and degraded airport facilities. In October 2014, an Airbus A330-300 crew at the end of a transatlantic flight planned a benign arrival into Montreal Trudeau’s (CYUL) Runway 24R in VMC with light winds and only a few cumulonimbus clouds in the area.
As the flight progressed, conditions began to rapidly deteriorate; the crew began to note an area of “strong returns” on the weather radar to the north of the airport. Soon, discussions on the flight deck switched from landing with a “light wind” to preparing for a possible encounter with windshear. Once established on final approach to Runway 24R, ATC gave an updated wind report as 280 degrees at 13 knots gusting to 18 knots and a reminder that the runway lighting was out of service.
According to the report, the flight crew never lost visual contact with the runway and the approach was stable. Passing through 900 feet agl, the first officer—the pilot flying (PF)—disconnected the autopilot. Seconds later, the preceding aircraft, a Bombardier DHC-8 reported “light windshear below 400 feet.” ATC then advised the Airbus crew of a wind increase—“300-degrees at 18 knots, gusting to 24.” Next, the pilots observed a rain shower approaching the middle of the runway.
At 130 feet agl, the aircraft entered a heavy rain shower. Flight data indicated roll oscillations to the left and right (later described as pilot-induced oscillations) down to about 50 feet agl, where the aircraft crossed the runway threshold on the centerline. Thrust was then reduced to idle at 30 feet agl with the aircraft in a left bank. The aircraft quickly began to drift to the left of centerline.
Next, the pilot simultaneously rolled the aircraft to the right and flared. The aircraft touched down on its right main landing gear (MLG) approximately 55 feet to the left of the runway centerline.
At touchdown, the aircraft was “crabbed” eight degrees to the right of the runway heading. Within seconds, the left MLG clipped several runway edge lights and touched down in the grass at the runway edge. Afterwards, the aircraft began to decelerate and then corrected back towards the runway centerline and exited the runway.
The investigation concluded that strong precipitation to the north of the runway created a temporary drop in temperature and change in wind direction; this confirmed the presence of a downburst from a thunderstorm cell at the time of landing. Contributing to the runway excursion was the increased intensity of the precipitation causing reduced visual references in the flare.
Furthermore, a lack of runway lighting made it difficult for the pilot to detect the lateral movement of the aircraft over the runway. The A330-300 has both reactive and predictive windshear systems, each are inhibited below 50 feet agl. Likewise, the airport did not have a low-level windshear system installed. Two aircraft on approach behind the A330 both discontinued their approaches due to deteriorating weather conditions.
A recent Airbus study identified three main environmental factors that may contribute to a runway excursion during landing: wet or contaminated runway, turbulence or crosswind, and degraded visibility.
The study concluded that 88 percent of runway excursion events involved a wet or contaminated runway, while nearly 80 percent of the events included at least two of the three environmental factors; the A330-300 runway excursion event in Montreal tagged all three.
Statistics point to handling issues as a significant contributor to runway excursion events during landing. Airbus and others recommend a holistic approach that “combines a variety of dimensions.” These include information and awareness (the environment), state of mind and preparedness, and handling skills. Here we will focus on handling skills; something that every ATP-rated pilot should be able to control.
Generally, pilots should break down the landing technique into three phases: before the flare, from the flare to touchdown, and after touchdown or the rollout.
Before the flare is the best opportunity to discontinue the approach or go around. Paramount is an awareness of the landing conditions. Questions to ask: if there is a crosswind, are you within limits? If the runway is contaminated, is the braking coefficient sufficient for a safe landing? If the answer is “no” to either, do not attempt to land.
Accordingly, initiating a go around close to the ground or even after a bounced landing is better than an unsafe landing. If you are not stable at any point, go around!
Oddly enough, proper seat position and rudder pedal adjustments are important. Seat position, especially height, is a must for that consistent “sight picture” in the flare. Likewise, full rudder travel is always important to keep the aircraft on the runway during strong crosswinds, engine-out handling, and rejected takeoffs.
From the flare to touchdown during a strong crosswind landing probably requires more skill than any other maneuver in an aircraft. No pressure.
To “ace” this maneuver, the pilot must land in the correct zone, with the right alignment and the right amount of energy. In a crosswind, the pilot must maintain the proper lateral flight path.
In general, fly the aircraft wings level and crab into the wind maintaining the proper trajectory along the runway axis. The crab (on most aircraft) is removed just before the flare with a smooth choreographed transition to align the longitudinal axis of the aircraft with the runway using rudder (primarily) and aileron. Pilots of transport-category aircraft with underwing engines should limit roll to around five degrees (or aircraft flight manual limit) to prevent “pod strikes.”
Runway alignment is largely dependent on an understanding of lateral flight path alignment. On most aircraft, the localizer antenna is located under the radome in the center of the aircraft.
So, regardless of IMC versus VMC or autoflight versus manual flight, it really does not matter. In Airbus terms, “Correct lateral flight path means localizer centered or nose of the aircraft trajectory aligned with the runway axis.” All this really means is that in the transition from flare to touchdown, the goal is to keep the location of the LOC antenna or center of the flight deck aligned with the runway centerline.
There are a few gotchas to look out for in the flare maneuver—disconnecting the autopilot, going visual, or the “de-crab” maneuver in a crosswind. The first two are easy.
In general, the autopilot is doing you a favor. At autopilot disconnect, the aircraft should be trimmed—thus, avoid large control movements as the autopilot is disconnected.
Studies have shown that when pilots transition to visual flight there is a tendency to start a “de-crab” maneuver to align with the runway. In a crosswind, prematurely removing the crab will allow the aircraft to drift to the leeward (or downwind).
Other cautions include the flare technique or maneuver during a crosswind. An extended or high flare not only significantly increases the landing distance but decreases the aircraft energy, making it more sensitive to the crosswind. If the aircraft begins to drift excessively during the flare, go around.
After touchdown (rollout) it is imperative to continue to “fly” each control surface until you vacate the runway. Remember, the aircraft will rotate around its c.g. during rollout if not counteracted with the rudder.
Touching down in a crab creates a pivoting moment around the vertical axis by the combined effect of the lateral friction of the tires and inertia force applied at the c.g. The intensity of the pivoting moment is dependent on the runway surface friction.
Weathercock effect is another undesirable outcome of a crosswind landing. The effect of the wind on the vertical stabilizer causes the aircraft to yaw into the wind. Again, this is counteracted by using the rudder.
As aircraft speed decreases, the rudder becomes less effective. Differential braking may be required to steer the aircraft during strong crosswinds. Some aircraft require forward pressure on the yoke during rollout to ensure nosewheel effectiveness.
There is one last gotcha on aircraft certified for an autoland. On many aircraft, the autopilot controls the rudder surface, but the rudder pedals remain neutral. Before disconnecting the autopilot during rollout, the pilot must be ready with feet positioned on the rudder pedals and be prepared to immediately respond with large inputs on the rudder. Autobraking is recommended.
Runway excursions continue to be a threat during the landing maneuver. Pilots must constantly evaluate environmental factors such as a strong crosswind or contaminated runway surface before attempting an approach and landing. Awareness and even avoidance may be the best strategy to mitigate this risk.
Aircraft handling skills can only be improved through training and experience. Most of all, pilots should understand not only the aircraft limitations but their own personal limitations—when factors begin to “pile up,” it might be safer to just go around.
Pilot, safety expert, consultant, and aviation journalist Stuart “Kipp” Lau writes about flight safety and airmanship for AIN. He can be reached at [email protected].