Eye to Wheel Height - Coordination between “ILS”, “PAPI”, “Aiming Point”

Do you know what does eye-to-wheel height stands for?

Lets together!

This is a concept that all aviator should be familiar since many accidents as we all know labeled as preventable are reported due to its probably cause on its final conclusion reasons that includes; either the pilots were not familiar with this procedure or they did but chose not to follow it (mostly cases shows low appch profile). That said, I found a way to put it down to the table.

Therefore I want to demonstrate here something I learned, how to determine the installation distance by drawing. We could use CAD software. The results will be very accurate, and we can also verify with our eyes what we are doing!

Hope you all find itt useful.

Coordination between “ILS”, “PAPI”, “Aiming Point”

First, I would try to mention something about coordination between “ILS”, “PAPI”, “Aiming Point”.

Runway Category I, Category II, Category III, are all runways with an Instrumental Precision Approach. Therefore the three different systems: “ILS”, “PAPI” and the horizontal signs “Aiming Point” have to be coordinated together, in order to give the same information to the pilot.

All the three systems have to be in according with the same data:

• approach slope
• “MEHT” minimum eye height over the threshold

Approach slope

Usually the slope path has the value of 3°. Different angle of slope path can be adopted, according with the environmental condition, like obstacles on the landing final approach and so on.

“MEHT” minimum eye height over the threshold

About MEHT [minimum eye height over the threshold], we have to refer to the ICAO (See below)

The MEHT [minimum eye height over the threshold] is the sum of MWC [Minimum wheel clearance] and EWH [Eye-to-wheel height] of airplane in landing configuration. (See belo)

MEHT = MWC + EWH

In the first illustration we can see 4 different groups of “Eye to wheel height”, which depend by the type of aircraft.

To decide which group choose, we have to refer to the ICAO Aerodrome Design Manual.

Let’s have an example.

Suppose we have to land on the runway with a Boeing 747-400. On the ICAO Aerodrome Design Manual (unfortunately I don't have the illustration here but you can easily find it on dad google), we can determined 40,3ft for pilot’s eye above wheel with 3.0 degree of glide slope.

On the first figure, we have to choose the fourth group, EWH [eye-to-wheel height of airplane in the approach configuration] 8m up to but not included 14m. The MWC [Minimum wheel clearance] for this group is 6,00m.

About EWH[eye-to-wheel height of airplane in the approach configuration] we have to choose EWH = 14m in favor of safety. In that way all the aircraft with 8 < EWH > 14 are verified.

Therefore:

• EWH = 14,00m
• MWC = 6,00m.

Then, the MEHT is:????MEHT = MWC + EWH = 6,00 + 14,00 = 20,00m , are we fine?

With the data above we can calculate the installation distance of PAPI system, ILS system and “Aiming Point” signs.

On the internet we can find lots of literature for the mathematical calculation of the installation distance. The best, in my opinion, it is the Advisory Circular AC 302-009 issued by Canada Transport.

Here goes what I want to demonstrate here, how easy is to determine the installation distance just by drawing. It makes the results very accurate, and we can also verify with our eyes what is running.

I’ll show an example from my calculation for the rehabilitation of a random RWY with the following values.

The runway data are:

• Runway Length???????????????????????????????????????????3.901,87m
• Threshold 16L Height from sea level?????????????????4,19m
• Threshold 34L Height from sea level??????????????1,75m

Suppose we have to land on the runway with a Boeing 747-400. We can use the same data determined at the beginning of this the article.

• EWH = 14,00m
• MWC = 6,00m
• MEHT = 20,00m

We can apply the above data to the figure aboce. In that way we obtain the figure below.

Now we can draw a line crossing the “eye of pilot” point with a slope of 2° 43’. We name this line “minimum path slope”. Please do not take in consideration the path slope 3°, we’ll see after how to determine that.

Now we have to draw an horizontal line which represents the altitude of the “Conventional Optical Reference” of PAPI system from sea level.

The Conventional Optical Reference” altitude is the sum of the “ground altitude” and the “PAPI Optical Center” from the ground. For “PAPI Optical Center” we have to refer to the data sheet of the PAPI equipment. According to the “Aerodrome Design Manual'' : “PAPI units should be the minimum practicable height above the ground and not normally above 0.9 m. All units of a wing bar should ideally lie in the same horizontal plane …”

The intersection between the “minimum path slope” and the “Conventional Optical Reference” represents the distance of installation of the PAPI system. We name this point “Do”.

Now we can draw a line crossing the “Do” point with a slope of 3°. This line represents the Normal path slope 3°.

To find the right position of the GP [Glide Path] antenna of the ILS System we have to determine the TCH [Threshold Crossing Height]. According to the FAA (Siting Criteria for Instrument Landing System) the TCH may vary from 40ft to 60ft. The typical value of TCH is 55ft (16,76m).

Then we can draw a line crossing the TCH point, the name of this line is Glide Slope Beam 3°. To verify the coherence between the PAPI system and the ILS system we have to move the GP Aerial of the Aircraft onto the GP Beam line and verify if the eye of the pilot is still into the PAPI path corridor. We have to do that for each type of aircraft we want to use for the runway.

In order to find the right position of the GP antenna, we have to draw the ground surface around the predictable position. The intersection of the “Glide Slope Beam 3°” and the ground surface is the GP antenna position.

That's it guys!

Should you have anything to add, do not hesitate to comment below.

cheers!

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