This is a classic mishap where aiming for brick one leads to bad results.
Ever since one of my Air Force Boeing 747 squadron mates put eighteen wheel prints in the overrun at Andrews, the math behind aircraft deck angle has been a priority for me. What the Canada Transportation Safety Board calls "Eye-to-wheel height" really misses the point. What pilots really need to understand is how far behind their aim point the wheels will contact the runway. The geometry of the aircraft on approach fundamentally changes the point at which the wheels will touch in relation to the pilot's aim point. The pilots were experienced in the Challenger 604, which has an abnormally flat deck angle on approach. In that airplane, they were able to successfully aim for 500 feet down the runway and land fairly close to that target. In an airplane with a higher deck angle, such as the incident's BD-700, this is not possible.
In this model of BD-700 the main gear are 41 feet behind the pilots, but the wheels will touch 285' behind the pilot's aim point if the pilot does not flare. Hard to believe? See Deck Angle for the math.
Everything here is from the references shown below, with a few comments in an alternate color.
Photo: Aircraft Damage, from Transportation Safety Board of Canada, Aviation Investigation Report, Photo 4.
There are few contradictory ideas here. They computed a landing distance of 4300 feet, which would have included a 50 foot threshold crossing height, yet they planned on crossing the threshold below that height. They planned a firm touchdown, but selected the lowest setting of the autobrakes.
The geometry of approach at the 50 foot call would indicate a 3.45° glide path — arctan (50 / 830) — if aiming for "brick one," but 2.33° — arctan (50 / 1230) — if aiming for the 500' point, as the pilot had intended. Either way, the approach could have worked under these circumstances, but there was no margin for error.
Photo: Aircraft eye-to-wheel height, from Transportation Safety Board of Canada, Aviation Investigation Report A07A0134, photo 5.
The mishap aircraft was the longest aircraft the captain had ever flown, it is unlikely he would have ever been taught about the distance between his aim point and wheel no-flare distance. His most formative jet experience was in the Challenger 604, which has an unusual landing geometry.
[Transportation Safety Board of Canada, Aviation Investigation Report A07A0134, §1.9.4] C-GXPR's EWH calculations were completed using the actual conditions, an aerodrome at sea level, a 3.0° flight path and the landing gear fully extended. For the occurrence flight, the EWH was determined to be 16.35 feet. Based on the Global 5000 maximum landing weight of 78,600 pounds, the greatest EWH for the Global 5000 in the approach configuration was determined to be 17.2 feet.
[Transportation Safety Board of Canada, Aviation Investigation Report A07A0134, §1.9.5] Calculations were also completed by the manufacturer to obtain the EWH of the Bombardier Challenger 604 (CL604), assuming a 3.0° flight path, in both the light landing configuration and the heavy landing configuration. The EWH in the light landing configuration was calculated using a landing weight of 28,000 pounds and was determined to be 12.11 feet
The difference in eye-to-wheel height of the two airplanes is a mere 5 feet. That might explain the difference between just barely clearing the runway's edge and hitting the runway's edge in this particular case, but it doesn't explain the reason the pilot's 500 feet aim point is inappropriate. We can do better. . .
Figure: BD-700 deck angle, from Eddie's notes.
The investigators failed to consider the distance between the pilot's aim point and the theoretical point the wheels would touchdown without a flare, based on that aim point. Because of the aircraft's positive deck angle and the height of the pilot's eyes above the wheels, the distance is not one-for-one. Few manufacturers publish their aircraft deck angles but it can be estimated. In the case of the BD-700, using a photo of the aircraft on approach in the accident report we can determine the BD-700 has an approach deck angle of 5°, which is typical of most passenger jets. (The pilots were most recently qualified on a Challenger 604, which is a notable exception to this average. The 604 has an approach angle which appears to be very close to zero.)
Figure: BD-700 deck angle, from Eddie's notes.
If a BD-700 pilot flies an on-speed 3° glide path toward an aim point on a runway and does not flare, the aircraft's wheels will touch the runway 285 feet behind that aim point. In the case of this mishap, the pilot was on track to do just that, but when the throttles went into the retard mode the aircraft was not over the runway and the pilot's pitch feel would have been different because there was less ground effect. The pilot failed to compensate and the airplane dipped below the glide slope he had set up.
The lesson should be clear: your wheels will touch far behind your aim point unless you flare consistently. You will be hard pressed to do that unless your airplane is over the runway when you flare. Aiming short of a normal 750' to 1,000' aim point removes all margin for error.
More about this: Aim Point versus Touchdown Point.
Figure: Aircraft Damage, from Transportation Safety Board of Canada, Aviation Investigation Report, Photo 7.
Don't you just hate it when old Air Force pilots talk in jargon as if everyone has the same set of experiences? I try very hard to avoid that but it has been pointed out I failed with regards to "brick one."
"Brick One" refers to the very first inch of pavement on a runway. I don't know where that comes from, other than I heard it a lot in the Air Force.
Transportation Safety Board of Canada, Aviation Investigation Report A07A0134, Touchdown Short of Runway, Jetport In., Bombardier BD-700-1A11 (Global 5000) C-GXPR, Fox Harbour Aerodrome, Nova Scotia, 11 November 2007
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