What is the definition of Height Above Threshold (HAT)?

Question

I could use some help settling a disagreement about the definition of HAT.

So my company uses the word HAT to mean “height of the aircraft above TDZE…at any given distance…at any given time”

So as an example, on an approach if you’re 20 miles out at 6000’ … then your HAT = 6000’ (assuming TDZE=0). Then if you’re 10 NM out at 3000’ then your HAT = 3000’.

So based on my understanding of the AIM’s definition of HAT, the above example is an incorrect use of the word HAT. HAT is a charted number on a plate, and it’s fixed. For the individual approach being flown, HAT does NOT change as the aircraft altitude changes. It does not reflect the aircraft altitude above TDZE, any ANY given distance.

Am I wrong?

Answer 1

The height above touchdown (HAT) is the height of something (e.g. the aircraft, or a procedural height) measured relative to the elevation of the touchdown zone. Note that this is similar to but crucially different from above ground level (AGL).

When listed as AGL, the height is measured relative to the elevation of the underlying terrain, and thus the position of the aircraft over the non-flat terrain plays a role in the conversion between MSL and AGL. When converting between MSL and HAT, the terrain profile does not play a role.

The decision height on the map is given as HAT, not as AGL.

Suppose there is an up-slope in the terrain towards the threshold of the runway. The decision height is at 200' HAT, but due to the lower terrain on short final, this decision height will be crossed at 300' AGL (as measured by the radio altimeter).

Answer 2

HAT is published on instrument approaches to show the AGL altitude the aircraft would be at when crossing the MDA or DA.

Your company is using the HAT concept to help determine if they are on a standard glidepath or 300 FT/NM + HAT. On a visual approach, this would be a good method as a backup on whether or not they are trending high or low.

Interestingly enough, non-precision approaches in Europe use a similar concept with charted altitudes every mile to stay on a certain glidepath.

Answer 3

Your question references "Height Above Threshold" (see the link below regarding HAT vs HATh) -

Height Above Threshold (HATh) is the height of the DA (decision altitude ) or the MDA (minimum descent altitude) above the Threshold of the runway published for a straight-in instrument approach.

Another way of describing HAT, but meaning the same as noted above (from the perspective of Decision Height [DH] vs DA [Decision Altitude] from the Pilot/Controller glossary in the FAA's Aeronautical Information Manual: (see the link below for information regarding HAT - Height Above Touchdown versus HATh - Height Above Threshold)

HEIGHT ABOVE TOUCHDOWN (HAT)− The height of the Decision Height or Minimum Descent Altitude above the highest runway elevation in the touchdown zone (first 3,000 feet of the runway). HAT is published on instrument approach charts in conjunction with all straight-in minimums.

This height does not change based on the aircraft's position or altitude.

From the FAA Chart User's Guide -Terminal Procedures Publication:

Note the link below: Change 20 (dated 12/2007, see Para 4 a. (1) [b] of the explanation of changes) for FAA's guidance replacing the term HAT (Height Above Touchdown [touchdown zone elevation -TDZE]) with HATh (Height Above Threshold).

https://fsims.faa.gov/wdocs/orders/8260_3b-chg20.htm