Threshold Crossing Height, how do you work it out?

[Metro Boy]

On an Australian T Vasis in a 737 you fly it with no dots up or down to be on slope. How do you work out how many dots to fly down for say a 747?

[Ultralights]

im not sure about the actual height, but i would guess about 3 deg angle from the touchdown markers, and im pretty certain that no matter what type you fly, it will always be the same dots up or down. a 3 degree approach is the same for a cessna 152 and a B 747.

[hoss]

Good question, was thinking of this the other day while watching a 'heavy' float half way down the runway. Also begs the question if they fly 1 or 2 dots high for a visual approach then how do they go coupled all the way to the ground? Why change, and if flown 'onslope' where do the mains touchdown? Allowing for float would have thought it would be close to the TDZ.

hoss:)

[OZBUSDRIVER]

AIP AD1.1-30 section 5.1.1

T-VASIS yadayada... Increased eye height over the threshold can be achieved by flying the approach with one or more of the "fly-down" lights visible.

3 lights fly up= 0 to 7 ft
2 lights fly up= 7 to 25ft
1 light fly up=25 to 41ft
ON GLIDESLOPE=49 feet
1 light fly down=57 to 75ft
2 lights fly down=75 to 94 ft
3 lights fly down=94 to176ft

[18-Wheeler]

On an Australian T Vasis in a 737 you fly it with no dots up or down to be on slope. How do you work out how many dots to fly down for say a 747?

We normally fly the t-vasis down to a few hundred feet, then let it drift up a little, about one to two dots.
Aim for a crossing height of 50', and have the touchdown at 1500' down the runway.

We very rarely do a visual approach though.

[DirectAnywhere]

Problem with the T-VASIS is it's not a "point source" aid therefore a large variety of approach heights will give an "on-slope" indication. Being 20' high crossing the threshold will result in landing 400' further down the runway.

With the aim of the main wheels crossing the threshold at 36' and eye height being roughly 40' above that, typically looking for two dots fly down, although that will give a range of eye heights anywhere from 74 to 94 feet as indicated and main wheel heights 34 to 54 feet.

Basically PAPI's a lot better aid for long bodied aircraft with eye height variation of only 7' per dot.

Books say use the aid as a guide to not less than 300' HAT then discard and use visual aimpoint from there.

When flying a "coupled" approach, the aircraft isn't coupled to the VASI. Obviously, it's coupled to the ILS which is designed appropriately, studied by far better brains than mine and published as an autoland runway. I always check the TCH published for the ILS is checked prior to commencing the approach to ensure sufficient main wheel clearance at the threshold.

In a 747 there are 2 glideslope antenna located in the radome and nose gear door area - not sure exactly where - but when the gear is extended the antenna transfers from the radome to the gear door which is roughly 20' above the main wheels. Therefore, minimum TCH for the G/S of 48' is acceptable, resulting in main wheel height of 28' at the TH. Anything below that requires visual adjustment of the aimpoint to ensure sufficient threshold clearance is available.

[Metro Boy]

Directanywhere, if you had a TCH of 50' would you fly two dots fly down? What if the TCH was 75'? Is that three dots flydown?

[DirectAnywhere]

Sorry, I should have mentioned that TCH isn't strictly correct terminology for a VASIS as, unlike an ILS, a range of eye heights over the threshold will result in an "on-slope" indication.

The term "Minimum Eye Height" or MEHT is used as it indicates the minimum eye height at the threshold that will provide an "on slope" indication.

For a standard T-VASIS or AT-VASIS, to achieve eye height over the threshold of 50' you would fly an on-slope indication. In a jumbo, this would never be done though, as it results in unacceptable main wheel clearance over the threshold of about 10'.

To achieve eye height of 75' over the threshold you would just want to see the second fly down light starting to become visible. This would result in main wheel clearance of 35', which is typical of the target we're looking for.