I first read about and adopted the Jacobson Flare technique as a young private pilot, while eagerly building my hours up before taking on a commercial pilot licience. The year was 1987.
(In fact it was an article published in the Australian "Aviation Safety Digest" no.134 (Spring 1987). That edition was solely dedicated to the subject of landing. A post by Checkboard in the other thread highlighted by sander82 in this thread includes a copy of that very article. Unfortunately it is missing the diagrams which help explain the concept alot.)
Frankly i consider the Jacobson Flare to be THE best aviation tool I have ever learned! I can not recommend it highly enough. It has allowed me to flare and land every aeroplane I have flown, first time, every time, with confidence and consistancy! As an example, I remember using the formula on the B767-200 and the first time I flew the simulator, I landed it without any imput from the instructor. No floating, no over control, no doubt! Same thing again when I transferred to the B737.
019360 says:
"Fly a stabilized approach,looking more and more outside as you get near the ground. At about the right height (visual cues and radio alt) you flare and land. Never been different."
This is EXACTLY the sentiment Captain Jacobson is addressing. An excerpt from his article:
"Historically, instruction in determining a suitable and consistent flare point has been inadequate to say the least. We are attempting to recognize and extract one flare point from a range of acceptable flare circumstances. Generally, the best that instructors have been able to manage is to demonstrate a suitable flare point for a particular aircraft as being 'about here'.
The student pilot has no proper model except his memory, and that in itself is inconsistent. Trial and error are the arbiters in determining the soundness of his developing judgment. Unfortunately even after the basic skills are mastered, the problem still exists because every aircraft type requires a different flare height. as a pilot converts to successive aircraft types, he faces the same problem over and over. He has no proper model at the very time he needs one most, and there lies a clue."
The problem with "At about the right height (visual cues and radio alt) you flare and land." as espoused by 019360 and others is that the visual picture changes with runway slope and especially runway width. A 30 metre wide runway look very different to a standard 45 metre wide runway, which looks very different to a 60 metre wide runway.
At the correct flare height, the narrow 30 metre wide runway makes it look as though you are still too high to commence the flare. Invariably those waiting for the "right" visual cues or picture (based on a 45 metre wide runway) will flare too late and a very firm landing will often result.
The opposite happens if you are landing on a 60 metre wide runway. Again if you are waiting to see the right picture based on a 45 metre wide runway, you will invariably begin the flare too early, and a long landing will result.
The runway width is irrelevent to someone using the Jacobson Flare technique.
When you get down to it, the concepts are very basic and quite brilliant! You simply fly your eyes towards your chosen aiming point on the runway ( usually somewhere near the 1000 ft markers (300m from the threshold). When a pre-determined reference point on the runway centreline passes behind/beneith the aircraft's glareshield, that is the time to commence the flare. (This reference point is actually the point on the runway where the wheels would have hit the runway if there was no flare.)
It is brilliant because you are effectively determining the height above the runway to commence the flare without having to actually think about or try to estimate what the aircraft's height actually is! Instead it utilises basic triginometry ie "the branch of mathematics dealing with the relations of the sides and angles of triangles and with the relevant functions of any angles." so that you are only using horizontal distances along the runway instead. As a result, because the approach angle is 3 degress or 20:1, if you can discern a distance along the runway of 20 feet (6.1 metres) (about half the length of a single centreline marking) you can discern a flare height to within 1 ft!!!
Get it???
There are two formulas you can use. The first one is:
Reference Distance = [(60/Glide Path Angle) x (vertical height above mainwheels)] + horizontal distance of eyes from mainwheels.
For example: a light aircraft with the pilot's eyes 5 feet above the mainwheels and no significant horizontal distance between them -- on a 3 degree glide path angle:
Reference distance = 60/3 x 5 = 100 ft (30 metres)
So assuming you are flying an accurate 3 degree slope to your chosen aiming point on the runway, when a point on the runway surface 30 metres this side of your aiming point passes below your glareshield, it is time to tranfer your aiming point to the end of the runway and flare.
B747 = [(43 ft x 20) + 90 ft] = 950 ft
So if you are aiming for the 1500 ft markers, when the 500 ft markers disapear under the glareshield, it is time to flare.
Now this formula assumes that the glareshield cut-off angle is consistant between aircraft types, and apparenty they are remarkably similar.
There is another formula to refine this reference distance if you know the flare height desired by the manufacturers and you know the glareshield cut-off angle.
Again for the B747:
Height difference between pilot eyes and main wheels = 43 ft
Flare height above runway (main wheels) = 30 ft
Therefore eye height at flarepoint = 30+43= 73 ft.
Glareshield cut-off angle = 16 degrees.
Approach slope angle = 3 degrees.
Aimpoint distance from threshold = 1500 ft
Reference distance = 73 ft x (cot 3 - cot 16) = 73 x (19.08-3.49)
Reference distance = 1138 ft. Assume 1140 ft.
Now admittedly there is around 100 ft discrepancy between this distance and the first one. But if you divide 100 ft by 20, that is the actual discrepancy in terms of height. It's just 5 feet. At 750 ft/min that equates to 0.4 of a second! Split the difference if you like.
So 1500 - 1140 = 360 ft.
B747:When a point 360 ft beyond the threshold (140 ft prior to the 500 ft markers) passes below the glareshield and out of view, begin the flare.
For the
B767:
Height difference between pilot eyes and main wheels = 30 ft
Flare height above runway (main wheels) = 25 ft
Therefore eye height at flarepoint = 30+25= 55 ft.
Glareshield cut-off angle = 17.5 degrees.
Approach slope angle = 3 degrees.
Aim point distance from threshold = 1150 ft
Reference distance = 55 ft x (cot 3 - cot 17.5) = 55 x (19.08-3.17)
Reference distance = 875 ft.
So 1150 - 875 = 275 ft
When a point 275 ft beyond the threshold (225 ft prior to the 500 ft marker) passes below the glareshield, it is time to flare.
B737-400 we shall use the first formula.
Height between eyes and main wheels = 17 ft
Distance between pilot eyes and main wheels = 46 ft
Glide slope = 3 degrees.
Aiming point distance from threshold= 1000 ft.
(17 x 20) + 46 = 386 ft ( Assumed 390 ft)
Reference point =1000 - 390 = 610 ft beyond the threshold.
Seeing as the 500 ft marker is around 100 ft (30 m) long and begins at the 500 ft point, you can safely say that the reference point is at the other end of the 500 ft marker.
I know this post is a long one but I really do feel it is worth exposing. For those non-believers, I simply ask you to take note of these reference points . Next time you are support pilot / Pilot Not Flying just notice when that point passes under the glareshield and how close it is to when the flare begins. Have a think about how easy it is to see it happening. It doesn't require you to look anywhere other than the runway centreline. In fact it doesn't even require you to take your eyes off the aiming point, just peripheral vision is enough, otherwise it is a minor and quick glance away from the aiming point at the most.
Regarding the actual flare, there was another brilliant article covering that topic in the same edition of the Aviation Safety Digest magazine. It prevented overcontrolling, over flaring, and allowed the aeroplane to almost land itself. I might paraphrase that if anyone is interested.
Cheers.
