TDZE vs THRE

Velocity173

Velocity173

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Velocity173
I noticed most IAPs show a Touchdown Zone Elevation (TDZE) on the chart. I've also noticed a Threshold Elevation (THRE) depicted on a few as well. What criteria dictates when a TDZE is published or when a THRE is published? :confused:
 
Velocity173 said:
I noticed most IAPs show a Touchdown Zone Elevation (TDZE) on the chart. I've also noticed a Threshold Elevation (THRE) depicted on a few as well. What criteria dictates when a TDZE is published or when a THRE is published? :confused:
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TDZE is old. THRE is new. It changes when they update the chart.
 
Ron Levy said:
TDZE is old. THRE is new. It changes when they update the chart.
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I saw this as a test question for my IFR written while I was studying tonight... So you're saying the TDZE question will be on the instrument written test for the next 50 years? :lol:
 
Ron Levy said:
TDZE is old. THRE is new. It changes when they update the chart.
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Do they both have the same meaning? The highest point in the first 3,000 ft of the landing surface.
 
Velocity173 said:
I noticed most IAPs show a Touchdown Zone Elevation (TDZE) on the chart. I've also noticed a Threshold Elevation (THRE) depicted on a few as well. What criteria dictates when a TDZE is published or when a THRE is published? :confused:
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The touchdown zone is typically 1000-1500 ft beyond the threshold.
 
gismo said:
The touchdown zone is typically 1000-1500 ft beyond the threshold.
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Yeah I guess THRE is literally the elevation at the runway approach end. Just wondering why they are replacing TDZE with THRE.
 
Thought I read somewhere it was more standardization with Europe. We're used to "line up and wait" by now, I suppose.
 
This was discussed awhile back. The only legal issue I recall was the issue of an instrument approach where you get the approach lights and are allowed to go down to 100 feet above TDZE.

Did that ever get clarified that the two are interchangeable for this purpose? IIRC the reg just says TDZE and if your chart doesn't have one you may be in a grey area.
 
Captain said:
This was discussed awhile back. The only legal issue I recall was the issue of an instrument approach where you get the approach lights and are allowed to go down to 100 feet above TDZE.

Did that ever get clarified that the two are interchangeable for this purpose? IIRC the reg just says TDZE and if your chart doesn't have one you may be in a grey area.
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I don't think the FAA has thought about it, and I doubt it would be an issue other than on an ATP oral anyway.
 
Why ATP? Wouldn't that be an issue for a regular IR as well?
 
You certainly have more experience with it than me. Seems a legit question to me though...
 
The way this thread is headed it begs the question......

Which airport/runway has a diffence of more then a couple of feet in elevation between those two reference spots ?....

You need to be paying attention to landing the plane,,,not looking at the altimeter to see if it changes 1 foot... Geez ....

Here in JAC I am guessing the difference is maybe 3-4 feet. Like you can see that on an altimeter..:no:
 
It's a small issue, I agree.

But,

The issue isn't reading 1 foot an an altimeter. The issue is how do you continue past the DH with the approach lights in sight if a TDZE isn't published? The reg states TDZE only, IIRC.
 
Captain said:
It's a small issue, I agree.

But,

The issue isn't reading 1 foot an an altimeter. The issue is how do you continue past the DH with the approach lights in sight if a TDZE isn't published? The reg states TDZE only, IIRC.
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Use the best information available to comply with the 100-foot rule.
 
These issues of a couple feet difference have real world ramifications.

Every six months I take a checkride that includes a circling approach. We often use the LOC to 27 in MEM to circle to 18R. Our OpSpecs require a circle to be done at 1,000' (or published if higher). Our altitude preselecter goes in 100' increments, so you have to round up...never down.

So what do you use? The TDZE of 27 (the approach your coming in on) or the airport elevation. Does it matter?

Runway 27 TDZE is 292'. So that would give a 1,300' altitude to go in the preselecter to circle. BUT, your circling the airport, not the runway. The Airport Elevation is listed as 341'. That would give you an altitude of 1,400' to go in the preselecter.

If you circle at 1,300' you bust your circling mins. You violate OpSpecs and have some retraining in front of you.

I don't mind using THRE in lieu of TDZE, I just wonder if there's official guidance on the subject.
 
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Captain said:
These issues of a couple feet difference have real world ramifications.

Every six months I take a checkride that includes a circling approach. We often use the LOC to 27 in MEM to circle to 18R. Our OpSpecs require a circle to be done at 1,000'. Our altitude preselecter goes in 100' increments, so you have to round up...never down.

So what do you use?
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What do your ops specs say? But absent that, since circling MDA's have HAA, not HAT, I'd say use airport elevation. That's what TPA's are based on, too.
 
Ron Levy said:
What do your ops specs say? But absent that, since circling MDA's have HAA, not HAT, I'd say use airport elevation. That's what TPA's are based on, too.
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The part you quoted was me asking the question to set up the answer that I gave the the part you didn't quote. Yes, it's HAA.
 
Captain said:
The part you quoted was me asking the question to set up the answer that I gave the the part you didn't quote. Yes, it's HAA.
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Then why even ask the question, when we were talking about TDZE vs THRE, not TDZE/THRE vs airport elevation?
 
Lending to the larger point of how a few feet can and does sometimes make a big difference. In the case of MEM circle a mistake between TDZE and Airport Elevation can be a busted checkride.

You know me, I tend to make points by using extremes.

:)
 
Captain said:
Every six months I take a checkride that includes a circling approach. We often use the LOC to 27 in MEM to circle to 18R. Our OpSpecs require a circle to be done at 1,000' (or published if higher). Our altitude preselecter goes in 100' increments, so you have to round up...never down.
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You circle based on an AGL height rather than MSL?
 
Everskyward said:
You circle based on an AGL height rather than MSL?
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No, read it again.

We circle 1,000' above the airport elevation (unless published is higher) and put that into the preselecter as an MSL altitude to fly.

MEM is 341' elevation. So we circle at 1,400' MSL.
 
Captain said:
No, read it again.
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I did.

Captain said:
We circle 1,000' above the airport elevation (unless published is higher) and put that into the preselecter as an MSL altitude to fly.

MEM is 341' elevation. So we circle at 1,400' MSL.
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What I understand is that you compute your own circling altitude based on an AGL, not what is published as the circling minimums for your category of airplane on the chart.
 
That is correct...with the understanding that whatever derived altitude we come up with has to be higher than published. If not then we use published. Pretty standard stuff.
 
Captain said:
That is correct...with the understanding that whatever derived altitude we come up with has to be higher than published. If not then we use published. Pretty standard stuff.
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I thought standard was using the published altitude but I understand what you are saying.
 
I think what he was trying to say is the company manual calls for published MDA or 1000 HAA, whichever is higher.
 
Ron Levy said:
I think what he was trying to say is the company manual calls for published MDA or 1000 HAA, whichever is higher.
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Yes. It's been that way at the two 121 and one 135 companies I've been at. I'm pretty sure the VMC circling thing is more common than not. I've only heard of a couple places that use published circ mins.
 
Captain said:
Yes. It's been that way at the two 121 and one 135 companies I've been at. I'm pretty sure the VMC circling thing is more common than not. I've only heard of a couple places that use published circ mins.
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Now you've heard of another one.
 
Captain said:
These issues of a couple feet difference have real world ramifications.

Every six months I take a checkride that includes a circling approach. We often use the LOC to 27 in MEM to circle to 18R. Our OpSpecs require a circle to be done at 1,000' (or published if higher). Our altitude preselecter goes in 100' increments, so you have to round up...never down.

So what do you use? The TDZE of 27 (the approach your coming in on) or the airport elevation. Does it matter?

Runway 27 TDZE is 292'. So that would give a 1,300' altitude to go in the preselecter to circle. BUT, your circling the airport, not the runway. The Airport Elevation is listed as 341'. That would give you an altitude of 1,400' to go in the preselecter.

If you circle at 1,300' you bust your circling mins. You violate OpSpecs and have some retraining in front of you.

I don't mind using THRE in lieu of TDZE, I just wonder if there's official guidance on the subject.
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That is really a different issue. Circling minimums are required to be based on airport elevation. That has remained unchanged at least since 1967.
 
Okay, maybe a poor correlation. I didn't mean to open a can of worms.

My only real concern of the topic (TDZE replaced by THRE) is the continuing past the DH issue as I'm pretty sure the reg still only lists TDZE without authorization to use THRE in lieu.
 
Captain said:
My only real concern of the topic (TDZE replaced by THRE) is the continuing past the DH issue as I'm pretty sure the reg still only lists TDZE without authorization to use THRE in lieu.
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That's what the reg says, but Flight Standards tells me it's just a case of missing all the ramifications of a change elsewhere. It will be addressed as a technical correction on the next cycle by the branch responsible for that section. Meanwhile, Flight Standards isn't going headhunting over the issue, so like they say in Russia, "nyet sweat."
 
Ron Levy said:
That's what the reg says, but Flight Standards tells me it's just a case of missing all the ramifications of a change elsewhere. It will be addressed as a technical correction on the next cycle by the branch responsible for that section. Meanwhile, Flight Standards isn't going headhunting over the issue, so like they say in Russia, "nyet sweat."
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That's what I gather from when I called my local FSDO over this. I think my response is in the older thread. But just to refresh everyone's memory, it was considered a non-issue. The lady at the FSDO says that the FAA has no official ruling on this mismatch. But the unofficial word from the FSDO pilots was that they are interchangeable.

So as Ron said "nyet sweet"
 
Ron Levy said:
That's what the reg says, but Flight Standards tells me it's just a case of missing all the ramifications of a change elsewhere. It will be addressed as a technical correction on the next cycle by the branch responsible for that section. Meanwhile, Flight Standards isn't going headhunting over the issue, so like they say in Russia, "nyet sweat."
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Thanks Ron. If its good enough for you it's good enough for me.
 
Just back from ACF-IPG. I discussed this with the chairman over lunch yesterday and sent him a follow up email on the subject. There isn't a definition of the term in the PCG for THRE, at least that I can find, although it is decoded in the TPP front material Acronyms. If I understand AC 150-5300-3 correctly, the difference can be over 20 feet. I recall when we had our ILS installed, we did not meet the grade requirements and had to obtain a deviation because of the runway slope that affected the TDZE area.
 
John Collins said:
Just back from ACF-IPG. I discussed this with the chairman over lunch yesterday and sent him a follow up email on the subject. There isn't a definition of the term in the PCG for THRE, at least that I can find, although it is decoded in the TPP front material Acronyms.
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It is defined in TERPS.
 
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