There are many reasons why airplanes crash. Lift and drag are always connected, and perhaps never more so than when a plane enters a stall. That’s what happened on a cold, winter day in 2009 in upstate New York in the Colgan Air 3407 crash.
On a cold winter day in 2009 in upstate New York, Captain Marvin Renslow and First Officer Rebecca Shaw were at the controls of a Bombardier Dash 8 Q400 operated by Colgan Air. The flight was a short 52-minute hop from Newark, New Jersey to Buffalo, New York; unfortunately, the flight would never arrive at its intended destination.
As the aircraft approached the Buffalo airport, the weather conditions there was a wintry mix of light snow, fog, moderate winds, and a temperature of 1 degree Celsius. The captain and first officer noted ice accumulation on the wings and windshield as they approached the vicinity of the airport. After lining up with the final approach course, and lowering the landing gear and flaps, the aircraft’s speed began to decay more than normal.
As the autopilot worked to hold the aircraft’s altitude, suddenly the wing lost a lot of lift and the aircraft began to fall out of control. Only about 20 seconds after the first sign of trouble, the flight crashed nose-first into the ground, and the aircraft exploded into flames. Sadly, this incident resulted in the death of all 49 passengers and crew, along with one person on the ground.
Learn more: Stall Events and Lift Induced Drag
What Went Wrong?
Let’s talk about what happened, and how a routine flight could go so horribly wrong in just a moment.
The fundamental aerodynamic principle at work relates to how lift and drag work together on a wing to govern the properties of flight. Since the amount of lift generated by a wing is directly related to its angle of attack relative to the oncoming flow, higher angles give more lift. If the wing needs to generate a higher lift coefficient (for example, to fly more slowly), then the aircraft can be pitched up to a higher angle of attack. This is because the lift must remain equal to the weight, and as airspeed goes down, the lift coefficient must go up.
The primary way that a pilot can increase lift coefficient is by increasing the angle of attack of the wing as the nose is pitched up. This increases the amount of flow turning and increases the downward deflection of the wake downstream of the wing, and from Newton’s third law, the lift increases.
In the Colgan Air 3407 crash, accumulation of ice on the wings and windshield increased the aerodynamic drag. The presence of the ice disrupted the smooth flow of air over the wings, and the roughness of the ice led to much higher viscous drag. Also, the ice would likely lead to small pockets of separated flow, which would significantly increase the pressure drag.
The overall impact of this ice accumulation is a more sluggish aerodynamic performance that should be counteracted by increasing the throttle setting on the engines. Since the pilots did not add power to overcome this additional drag, the aircraft began to decelerate. The autopilot, which controls the aircraft’s attitude (its pitch, roll, and yaw orientation), continued to pitch the aircraft to a higher angle of attack in order to generate enough lift to maintain a constant altitude.
There is a limit, however, to how much lift can be generated by a wing, or how high the angle of attack can go, before significant problems are encountered. Beyond a certain critical angle of attack, the flow streamlines cannot successfully negotiate the aggressive curvature near the leading edge of the wing, and the streamlines depart from the wing’s curvature. In other words, the flow separates.
This flow separation event occurs because the boundary layer has insufficient momentum to counteract the adverse pressure gradient that the flow is working against. The stronger the curvature of the flow streamlines, the more intense the changes in pressure will be, and the harder it will be for the flow momentum to overcome the increase in pressure required to recover back up to high pressure at the trailing edge.
When a Plane Stalls
The most significant consequence of flow separation over a wing is a dramatic loss of lift, which is called stall. The onset of this flow separation can happen very quickly as the wing is pitched to higher angles. It’s a very grave situation in an aircraft since the aircraft will begin to fall when lift is lost during a stall!
Each aircraft has specific stall characteristics – there is a physical angle of attack and corresponding minimum speed at which it can fly – any lower flight speed will result in a stall.
This is precisely what happened in the case of the Colgan Air crash. The insidious thing about ice accumulation on the wings is that it CHANGES the stall characteristics of the airplane, often in unpredictable ways that a pilot may not recognize.
When ice accumulates on the wing, it changes the shape of the wing, adding increased roughness and curvature, making it much more prone to flow separation. Thus, the minimum flight speed at which an aircraft will stall can dramatically increase in icing conditions.
The Problem with Ice
Let’s look at ice on the leading edge of a wing. There is a resulting flow field around it. Clearly, the airflow is disturbed significantly and the wing is no longer aerodynamically clean.The problem, though, is that the accumulation of ice is extremely difficult to predict – the exact shapes that result from various conditions are a result of the complex interaction between three-dimensional aerodynamics, heat transfer, and the size, temperature, phase, and density of water droplets that interact with the wing. This is actually an active area of research to understand and reliably predict how ice accumulates, and inform strategies for mitigating the problem.
For the Colgan Air crash, the Dash 8 aircraft would normally be flown at an approach speed of 145 knots with no flaps, in order to be well away from the stall speed of 118 knots for a clean wing. However, in icing conditions, the limiting stall angle of attack decreases and the stall speed goes up by an unknown amount, due to the uncertainty of ice accretion. Thus, safe operating procedures in icing conditions require that the approach speed be set 25 knots higher than normal, at 170 knots, in order to add a margin of safety.