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HAT = "aircraft altitude above TDZE"...or DA/MDA?

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HAT = "aircraft altitude above TDZE"...or DA/MDA?

Old 28th Oct 2021, 06:39
  #1 (permalink)  
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Join Date: Sep 2021
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HAT = "aircraft altitude above TDZE"...or DA/MDA?

Hello all,
Can someone help me settle a disagreement please about HAT?

So my 135 company uses HAT in our manuals to mean "height of aircraft above TDZE". And I get it. The letters HAT stand for "height above touchdown" ... so one would kinda expect the meaning of acronym to mean "height above touchdown". I get it. It's understandable. So in other words, my company basically uses HAT to mean "your aircraft altitude at any given point above TDZE". So for example - if you're 6000' above an airport at sea level, then your HAT is 6000'. And if you descend down to 3000', then your HAT is now 3000'. And so on. To put it another way, my company's use of HAT implies that HAT is variable. It changes with aircraft altitude. It's not a fixed value. As the aircraft ascends/descends, the HAT value changes. HAT varies. That's how my company uses HAT.

But here's my problem. The AIM indicates that HAT is "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)." It's a DH/MDA. It is NOT a measurement of aircraft relative altered to the airport at any given distance. So in other words, HAT is a DH or MDA (depending on the approach - let's just stick with ILS for sake of simplicity). So for an ILS, HAT is DH. DH on an ILS is a fixed value. It doesn't vary.

And by my logic, HAT does not vary. According to the AIM, it's a fixed value on a plate. It doesn't change with aircraft altitude. The HAT of an ILS (cat I) is 200'. If the aircraft is 6000' above TDZE (on a cat 1 ILS), the HAT is still 200'. If the aircraft is 8000' above TDZE, the HAT is still 200'. If the aircraft is 500' above TDZE, the HAT is still 200'. Ad nasueum.

But when I try to explain this to a check airman at my company, he just tells me I'm wrong. I've tried so many different ways to explain to him that the context of HAT in terms of how our company uses it is not the same context in which the AIM uses it.

Soooooo many different ways I've tried explaining it from different angles. For example, I even tried pointing out that the formula for VDP calculation is VDP=HAT/300. Well...there can only be 1 VDP per approach. If VDP is fixed, then HAT is fixed. If there's only 1 VDP, then there's only 1 HAT (per approach). And then I pointed out that if I use their definition of HAT...meaning that HAT varies with aircraft altitude...then you would constantly have numerous different VDP values as the aircraft ascends/descends...which makes no sense. As an example, using their definition of HAT (aircraft altitude above TDZE), if you're 6000' above TDZE, then my company says your HAT is 6000', so then your VDP would be 20 NM (6000/300 = 20). A VDP 20 NM away?! That's absurd. Why would a VDP ever be 20 NM away?! It's just ridiculous. The math on this example seems so simple to understand, and it seems like easy proof to verify HAT does not equal aircraft altitude above TDZE. This math seems like such simple logic to understand, yet no one at my company is picking up what I'm laying down. Feels like I'm banging my head against a wall.

Am I interpreting the AIM wrong? Any and all comments are welcome, of course. My goal isn't to prove I'm right. I just want an answer that makes sense. If I'm wrong, I'd simply like to understand why and have my logic invalidated with explanations that make sense (as opposed to just simply being told I'm wrong)

So all comments are welcome please. However, if a DPE could chime in...or someone who would be a particularly noteworthy/trustworthy authority on the matter...then I'd be enormously grateful for your input please.

Many thanks in advance!

Last edited by CA_flyer12; 30th Oct 2021 at 00:15. Reason: Poor word choice
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Old 29th Oct 2021, 19:10
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Here is a simple diagram

Last edited by Whopity; 29th Oct 2021 at 19:27.
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Old 29th Oct 2021, 23:14
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UNITED STATES STANDARD FOR
REQUIRED NAVIGATION PERFORMANCE
(RNP) APPROACH PROCEDURES WITH
SPECIAL AIRCRAFT AND AIRCREW
AUTHORIZATION REQUIRED (SAAAR)

1.4.13 Height Above Touchdown (HAT).
The HAT is the height of the DA above the highest point in the first 3,000 ft of the landing runway (touchdown zone elevation). See figure 1-5


This is the official wording and therefore the definition. If your company wants to add to its SOPS, as it sees fit, then it should find its own term to describe its own standard. These things are there for a purpose and should not be confused. If you haven't done so already, show him the above document.
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Old 30th Oct 2021, 00:14
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Yep

Originally Posted by Fl1ingfrog View Post
UNITED STATES STANDARD FOR
REQUIRED NAVIGATION PERFORMANCE
(RNP) APPROACH PROCEDURES WITH
SPECIAL AIRCRAFT AND AIRCREW
AUTHORIZATION REQUIRED (SAAAR)

1.4.13 Height Above Touchdown (HAT).
The HAT is the height of the DA above the highest point in the first 3,000 ft of the landing runway (touchdown zone elevation). See figure 1-5


This is the official wording and therefore the definition. If your company wants to add to its SOPS, as it sees fit, then it should find its own term to describe its own standard. These things are there for a purpose and should not be confused. If you haven't done so already, show him the above document.
Dude I did. I showed them screenshots of the FAA/AIM definition of HAT. I gave them the VDP formula. I gave them the screenshot you just posted. I gave them articles about how to interpret plates where the HAT is circled in the red in the profile view of the plate.

They still can’t follow me. They’re just stuck in the belief that HAT is used to describe your various aircraft’s various altitudes at any given point in space. They just don’t seem to grasp that (per approach each individual approach), HAT is a single, fixed (charted) number that doesn’t change with aircraft altitude.

So just to be clear…

You’re agreeing with me that they’re using HAT incorrectly, yes?
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Old 17th Nov 2021, 02:57
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You’re agreeing with me that they’re using HAT incorrectly, yes?
It would help to know how your check airman would teach it to you. Height and altitude are not interchangeable words, despite their similarities. It's the same thing as weight and mass. They're two very different, but similar, words, often used improperly. You've pretty much pointed out your correct understanding of the intent of HAT in your original post.

The reason I ask how your check airman would teach is that you state the definition in the manual is "height of aircraft above TDZE" which, although not 100% correct to the regulatory definition, would be correct if the intent of their phrase is to imply the DH (they should just change the wording in the manual if this is the case). One of the hardest types of discussions I have with other pilots is where we are both arguing the same point, but neither of us realizes it. Could it be that your check airman is arguing that the company means the DH, but because in discussion both altitude and height are used, that everyone is getting confused? I'm not saying the check airman is right. I'm a check pilot in Canada, and it's not a delegation that immediately confers instant knowledge of all things aviation, despite the facade we sometimes project. What I'm saying is that if you're both arguing the same point, it could be getting lost in syntax.

If it is really that bothersome and you think there is a safety concern, I'd file an SMS report if your company is one of the Part 135 operators that require an SMS. If not, is there a chief pilot or another check pilot you can talk to?
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