Aim Point vs. Touchdown Point
Takeoff and Landing Topics
We all agree that aiming for "brick one" is a bad idea and when we see someone else do it, we shake our heads and give a little tsk, tsk. Of course, when we aim for brick one, well that is just superior airmanship. Right?
I grew up aiming for brick one and once in the B-747 got that nonsense beaten out of me. There are several issues here:
- Runway Markings. What do those hash marks mean to you, the pilot? The answer depends on the size of the runway and there are minor variations depending on what country you are in. In just about every case on a runway long enough for a jet, the runway aim point will be around 1,000 feet down the runway and the touchdown zone markers occur every 500 feet.
- Look Down Angle. Where will your wheels touch the runway in relation to your aim point? (If you don't flare.) In a G450 the answer is 300 feet before the aim point for a 3 degree glide path. But the pilot's eyes are only 40 feet ahead of the wheels. How can this be? The answer lies with angles. More about this: Trigonometry for Pilots.
- Flare Distance. How much real estate should a flare take? Of course this depends heavily on pilot technique but there is an answer: it shouldn't take more distance than necessary. Some aircraft may specify a touchdown rate or other metric which makes the flight manual performance numbers possible.
Photo: KTEB Rwy 06 Flight Path Vector and 3° Line, from Eddie's notes.
Runway Threshold Markings
Figure: Runway markings, from Eddie's notes.
[Aeronautical Information Manual, ¶2-3-3.h.] Runway threshold markings come in two configurations. They either consist of eight longitudinal stripes of uniform dimensions disposed symmetrically about the runway centerline, as shown in [the figure], or the number of stripes is related to the runway width as indicated [below]. A threshold marking helps identify the beginning of the runway that is available for landing. In some instances the landing threshold may be relocated or displaced.
For narrower runways, the number of stripes can be as few as 4 for a 60 foot wide runway, or as many as 16 for a runway 200 feet or wider.
[ICAO Annex 14, Volume I, §5.2.4]
- A threshold marking shall be provided at the threshold of a paved instrument runway, and of a paved non- instrument runway where the code number is 3 or 4 and the runway is intended for use by international commercial air transport.
- The stripes of the threshold marking shall commence 6 m from the threshold.
- A runway threshold marking shall consist of a pattern of longitudinal stripes of uniform dimensions disposed symmetrically about the centre line of a runway as shown in [the figure].
Runway Touchdown Zone Markings
[Aeronautical Information Manual, ¶2-3-3.e.] The touchdown zone markings identify the touchdown zone for landing operations and are coded to provide distance information in 500 feet (150m) increments. These markings consist of groups of one, two, and three rectangular bars symmetrically arranged in pairs about the runway centerline, as shown in [the figure]. For runways having touchdown zone markings on both ends, those pairs of markings which extend to within 900 feet (270m) of the midpoint between the thresholds are eliminated.
[ICAO Annex 14, Volume I, §5.2.6] A touchdown zone marking shall be provided in the touchdown zone of a paved precision approach runway where the code number is 2, 3 or 4.
The code numbers refer to the size of the runway, where a Code 1 runway is less than 800 meters (2,625 feet) and the higher numbers are longer.
- A touchdown zone marking shall consist of pairs of rectangular markings symmetrically disposed about the runway centre line with the number of such pairs related to the landing distance available and, where the marking is to be displayed at both the approach directions of a runway, the distance between the thresholds, as follows:
|Landing distance available
or the distance between thresholds
|Pair(s) of markings|
|less than 900 m||1|
|900 m up to but not including 1,200 m||2|
|1.200 m up to but not including 1,500 m||3|
|1,500 m up to but not including 2,400 m||4|
|2,400 m or more||6|
Runway Aim Point Markings
[Aeronautical Information Manual, ¶2-3-3.d.] The aiming point marking serves as a visual aiming point for a landing aircraft. These two rectangular markings consist of a broad white stripe located on each side of the runway centerline and approximately 1,000 feet from the landing threshold, as shown in [the figure].
[ICAO Annex 14, Volume I, §5.2.5]
- An aiming point marking shall be provided at each approach end of a paved instrument runway where the code number is 2, 3 or 4.
- The aiming point marking shall commence no closer to the threshold than the distance indicated [below], except that, on a runway equipped with a visual approach slope indicator system, the beginning of the marking shall be coincident with the visual approach slope origin.
- An aiming point marking shall consist of two conspicuous stripes.
The location of the aiming point marking depends on the landing distance available. For a runway between 1,200 meters (3,947 feet) and 2,400 meters (7,874 feet) landing distance the aiming point begins at 300 meters (984 feet).
Look Down Angle
Your aim point, as your very first flight instructor told you, should be that point on the windscreen that doesn't move. That instructor would also add a lie: "If you don't do anything, that is where the airplane is going to first contact the runway." The problem with that last statement is that you have airplane underneath you, and as the type ratings accumulated, there was more and more airplane below and behind you to consider. If it were just your eyes doing the flying, then your aim point would indeed be the same as your touchdown point.
Where the airplane will touch down, then, depends on where the wheels are in relation to your eyes, the things doing the aiming. The answer varies with aircraft. We'll used the Gulfstream G450 as an example, but you can do the math for you airplane if you wish. Or you can just take away the bottom line message: don't aim for brick one.
Look Down Angle, G450 Example
Figure: G450 dimensions, side view, from G450 Aircraft Operating Manual, §2A-06-00, figure 2, part 2.
If you want to find the distance between where your eyes are pointed on approach and landing versus where your wheels are, you will need some dimensions. Specifically, you need the distance between the aft-most landing gear and the pilot's eyes. That may take some detective work or perhaps just a good tape measure.
In the case of the G450, the operating manual gives us the distance between wheels: 39' 1-1/4".
The G450 Weight and Balance Manual tells us the nose wheel has a horizontal arm of 48.1" while the pilot is at 41.0", 7.1" forward of the nose gear. Assuming the pilot's eyes are at the point used for the center of the pilot's mass, we can deduce:
We'll call that an even 40 feet.
Further research leads us to a Gulfstream Eye Wheel Height Paper which tells us the pilot's eyes will be between 10.4 and 10.8 feet off the ground, depending on weight and center of gravity. We'll use 10.5 feet.
Photo: G450 eye wheel height, from Eddie's notes.
The Gulfstream Eye Wheel Height Paper tells us our eyes will be 13.8' off the runway when the wheels touch on landing, which is what we needed for this exercise. The process to find the deck angle of the aircraft is shown for the sake of further discussion below.
Photo: G450 eye wheel height during landing approach, from Eddie's notes.
We can use basic trigonometry to determine the distance from our eyes to an aim point. In the case of a three degree glide path, our G450 pilot's eyes will be 263' behind the aim point, which means the wheels will be 303' behind the aim point. In other words, if you are aiming for the five hundred foot markers and don't flare, you wheels will just barely make the runway. Add a gust of wind or just a little deviation from glide path, you may not reach the runway at all.
Photo: G450 look down angle, from Eddie's notes.
Flat Deck Angle Impact on Look Down Angle Exercise
What about aircraft with flatter deck angles? The Challenger 604, for example, has 0° approach deck angle. To isolate only the deck angle factor, we will use G450 dimensions to explore the impact of deck angle on look down angle.
Photo: Flat deck angle impact on look down angle, from Eddie's notes.
As can be seen, aircraft with flatter deck angles will touchdown considerably closer to their aim points.
Aircraft Length Impact on Look Down Angle Exercise
What about longer aircraft? The G550 is 7'2" longer than a G450 and its wheels are 5'11" further apart. While the aircraft have the same eye wheel height when in a three-point attitude, the longer length translates to a higher pilot's eye position when the aircraft is in the landing approach attitude: 15.6' for the G550, 13.8' for the G450. This means the pilot's aim point will need to be further down the runway.
Photo: G550 look down angle, from Eddie's notes.
While not as dramatic an effect as the flatter deck angle, the longer aircraft does have a disproportionately longer distance to aim point. Adding a mere 7 feet to the length of the airplane increased the aim point distance by over 40 feet.
Glide Path Impact on Look Down Angle Exercise
A common tendency for inexperienced pilots is to shallow the glide path when trying to land as closely to brick one as possible. Experienced pilots know this is a fool's game because judging the wheel touchdown point becomes harder as the glide patch becomes shallower. As it turns out, this is proven by the math:
Photo: Glide path impact on look down angle, from Eddie's notes.
The round out from approach deck angle to landing flare is heavily dependent on pilot technique, to be sure. If you frequent runways short enough to require close examination of your landing distance charts, you ought to have a good idea how much runway your flare technique consumes. The Gulfstream G450 performance numbers, for example, are based on a very firm landing that you would be hard pressed to employ on a very long runway. But if you insist on those runway "grease jobs" you ought to know how to get the Airplane Flight Manual numbers when you need them. Of course I have a technique to remove the variability of my technique.
[G450 Aircraft Operating Manual §13-03-20] Landing distances based on 3.0° glide path at 50 feet and 6 FPS sink rate at touchdown.
A 6 FPS sink rate at touchdown equates to 360 feet per minute. That equates to cutting your approach sink rate by less than half; it is a noticeable arrival. I've found that cutting the descent rate to 100 feet per minute gives you a soft touchdown while consuming less than an additional 400 feet. If you consciously try to avoid flaring to level flight, instead aim for that 100 feet per minute sink rate, you will be able to get close to those AFM numbers and forever avoid a floating down the runway landing.
More about this technique: Landing / The Flare.
For example, a G450 flying a 3° approach at 125 KCAS will typically be sinking 600 fpm prior to initiating the flare, which should begin around 20 feet. Without a flare, the distance to touchdown from the 50 foot point can be found easily found with a little trigonometry:
If we want to start a flare at 20', which is appropriate in this airplane, we will arrive at that 20' point 572' from the threshold:
For the sake of the math, let's say we smoothly cut our descent from 600 fpm to my technique target rate of 100 fpm in a linear fashion, so that the average descent rate for the entire 20 feet comes to the average of (600 + 100) / 2 = 350 fpm:
The distance covered in 3.43 seconds at 125 KCAS:
Figure: 100 fpm flare technique, from Eddie's notes.
Using this technique in the G450, we really can land the airplane within 400' of charted numbers without having the jarring 6 FPS touchdown. Since I land the airplane this way every time, I simply add 400' to AFM numbers. The only variable left for me then will be thrust reverser and whell brake effort.
You can certainly verify your own results for whatever flare technique you use, by simply looking at the runway at touchdown, remembering to subtract the look down angle distance. Keep this distance in mind when you really need to land the airplane within the performance numbers in your manuals.
Portions of this page can be found in the book Flight Lessons 2: Advanced Flight, Chapter 16.
Gulfstream Eye Wheel Height Paper, GIVX-HF-06-01-11.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Weight and Balance Manual, Revision 3, March 2008
ICAO Annex 14 - Aerodromes - Vol I - Aerodrome Design and Operations, International Standards and Recommended Practices, Annex 14 to the Convention on International Civil Aviation, Vol I, 6th edition July 2013