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A few days back a McDonald Douglas MD-11 operating as FedEx Flight 80 crashed at Tokyo's Narita International Airport and resulted in the tragic deaths of both pilots.
So, what went wrong?
In order to understand what went wrong we need to cover how an aircraft is landed and surprise, surprise, it's a little more complex than one might think. First off is a list of the relevant technical terms and a diagram of the approach profile of an aircraft shown as the green dashed line and the runway from above that shows the markings.
- VS or Velocity Stall This is the speed at which the wings stall or cease creating lift. It usually results in a sudden drop of the nose and often results in the aircraft going into a spin that can be difficult to recover from.
- Threashold This is the beginning of the landing strip and is usually designated by parallel lines running across the runway.
- Aiming Point This is the point that the pilot aims the aircraft at during the approach. It is often marked on the runway by two large rectangles on either side of the runway. It can also be designated by a series of lights on either side of the runway called a Visual Approach System or Precision Approach System.
The approach and landing are broken up into five segments:
Glide Slope: Initially the aircraft is set up on the Glide Slope which is a straight line that terminates in what is called the aiming point that is usually designated by the wide set of lines on either side of the runway. The way you maintain the glide slope varies between aircraft types but surprisingly large jets and gliders do this in very much the same way. With the jet the engines are set to a specific glide slope power output and then as with the glider the angle of the glide slope is maintained using air breaks. Before the word police jump in the spelling of break is deliberate. Air breaks don't actually slow the aircraft down, they disrupt or break up the flow of air over the wing and therefore reduce the lift the wings are creating. The resultant reduction in lift increases the rate of descent and therefore increases the angle of the glide slope. From the pilots point of view maintaining a constant glide slope is fairly easy to do and is achieved by keeping the aiming point in the same relative position throughout the entire approach procedure.
Round Out: This is where things start to get complex as if the aircraft were to follow the glide slope all the way to the ground the impact due to the vertical speed would be too great for the landing gear and airframe to tolerate. This would result in the wheels and undercarriage being forced up through the wings and fuselage. Instead when the aircraft is about a wing span above the ground the pilot pulls back on the controls to arrest the vertical speed and set up the aircraft so it is flying horizontally. The exact timing varies from aircraft to aircraft but it is about this point that the engine power is reduced to idle.
Flare of Hold Off: This is probably the most important and difficult part of the whole procedure. At this stage the aircraft should be flying horizontally or parallel to the runway with the wheels a meter or so off the ground. Since the throttles have been reduced to idle the airspeed will start to wash off. Inorder to maintain the horizontal flight path the pilot gently pulls back on the controls. This gently raises the nose and increases the angle of attack on the wings which increases the lift and enables the pilot to maintain the horizontal flight path. This takes some practice to acchieve and needs to be a smoot increase in the attidude of the nose but every pilot alive has practiced this hundreds of times and should be able to do it without thinking.
Touchdown: Eventually the reducing airspeed and increasing angle of attack reach a point called VS where the wings can no longer create lift ant the wings will stall. This results in a sudden and dramatic reduction in the lift they are creating and the aircraft drops the last metre or so and the wheels contact the runway.
Roll Out: The aircraft's weight is now completely supported by the wheels and the aircraft is no longer flying. The pilots then use various braking systems like spoilers, wheel brakes, reverse thrust to slow the aircraft to a speed that it can safely be manoeuvred using the undercarriage rather than flight controls.
So, why such a convoluted technique? Why allow the aircraft to drop a metre or two onto the runway, surely a smoother approach would be to gently fly the aircraft onto the runway?
There's an very good reason for this procedure and it has to do with getting the aircraft to change from being an aircraft to a land craft in as little time as possible.
If you were to fly the aircraft onto the runway it would still be going fast enough to fly or faster than VS. If for any reason the nose wheel where to rise due to bouncing, hitting a bump, centre line marker etcetera the resultant raising of the nose would increase the angle of attack on the wings. Since the aircraft is still travelling faster than VS this would increase the amount of lift the wings create and as a result the aircraft would start to fly again and gain altitude. Since the engines are at idle this sudden increase in altitude washes off airspeed until it drops to VS at which time the wings stop creating lift. The nose of the aircraft then drops and the aircraft impacts the ground nose first with a vertical speed that is far greater than the airframe is capable of sustaining. The end result is a less than desirable landing and usually involves the destruction of the aircraft and those onboard.
Now that we have a basic working knowledge of how aircraft are landed we can apply it to the video of the FedEx Flight 80 crash. The three images below were taken from the video of the accident and show the aircraft slightly before, during and slightly after the aircraft first touched down.
If you look at the angle the fuselage makes to the ground the images show that it is increasing across the three images. This is exactly what we would expect to see during the round out, however, as is clearly shown by the puff of whit smoke in the middle and cloud of smoke in the right image the aircraft has touched down during the round out.
This is where things start to go dramatically wrong as it means the aircraft has touched down while still travelling faster than VS. This means that it will start flying again if the nose of the aircraft rises.
The next set of images show the aircraft shortly after it first touched down.
Unfortunately due to the tail of an aircraft in the foreground you cant see why but the sequence shows that for some reason the nose of the aircraft starts to rise. This means that since the aircraft landed and is still travelling faster than VS the increased angle of attack on the wings will create more lift and cause the aircraft to become airborne again.
The final series covers the time slightly before the second and major impact with the runway.
The first image of this series shows how the aircraft has become airborne and is now close to the angle at which the wings will stall and corresponds with VS. The second image is slightly later and shows how the nose has dropped dramatically due to the airspeed falling below VS and the wings stalling. The aircraft is now travelling too slow for the wings to fly and as a result the normal flight controls are ineffective. Since the aircraft is airborne it means that the normal ground handling controls are also useless and as a result the aircraft is completely out of control. The final image shows the second time the aircraft impacts the runway, but this time nose wheel first and with a vertical speed way in excess of the capacity of the landing gear and airframe.
We can now see what has happened, but like many answers it raises the following questions:
1. Why did the aircraft touch down with excess speed?
2. How could experienced pilots make such a mistake or misjudgement?
3. Since Narita International Airport is equipped with automated landing facilities why were the pilots flying the aircraft manually?
4. Was there some sort of mechanical failure in the aircraft that caused the aircraft to touch down prematurely?
5. Was there some sort of weather phenomenon that caused the aircraft to touchdown prematurely?
These are the questions the accident investigators must answer and it's beyond what can be achieved by just watching the video.
However, the meteorological conditions could have been a contributing factor and the pilots from aircraft that landed immediately prior to the accident reported encountering wind shear at around 2,000 feet.
The aircraft landed on runway 34L which means that the runway was aligned with a magnetic heading of 340°. The wind at the ground was coming from 320° so it was almost straight down the runway, but it was fairly strong at 26 knots (48 km/hr 30 m/hr) with gusts to 40 knots (74 km/hr 46 m/hr). This could definitely be a contributing factor as it could have caused a sudden drop in their airspeed of 14 knots which at approach speed could be devastating. However, pilots should allow for this when preparing to land by increasing the speed that they fly along the glide slope. This does make the round out and hold of more difficult to achieve, but atain, pilots are trained to do this and it shouldn't be a problem.
A final point that's worth mentioning is that FedEx flight 14 had a very similar accident at Anchorage International Airport on 31st July 1996, but fortunately all those on board escaped with only minor injuries.
This article is in no way meant to be a definitive answer to why the aircraft crashed but rather give the readers the background needed to understand what is happening in the video and stimulate discussion.
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