Part 135 Dest Alternate Ceiling question

[Bernoulli]

I have a part 135 question about the forecasted ceiling at your destination that would require you to file an alternate under part 135. This may seem to be an anal question at first glance… but the differences in interpretation can be huge.
Below is the reg:

Part 135.223(b)
(1)The ceiling will be at least 1,500 feet above the lowest circling approach MDA; or
(2) If a circling instrument approach is not authorized for the airport, the ceiling will be at least 1,500 feet above the lowest published minimum or 2,000 feet above the airport elevation, whichever is higher.

OK…Ceilings are reported in AGL (above ground level) above the airport… and minimum descent altitudes (MDA) are based on Mean sea level (MSL). So here lies the question…take this example:

Suppose XYZ airport is 5,000 feet above sea level. The lowest MDA is published as 5500 (500 Height Above Airport… HAA). So 5500 is the MDA… According to the regulation above, do I need to have a ceiling above 7000 feet (5500 MDA+ 1500 = 7000)?... Or should I take the MDA’s height above airport (HAA) which is 500 feet AGL, and add the 1500 feet to it, which would make the ceiling 2000 feet? The difference between 2000 feet and 7000 feet is huge.

Note…the reg does not say 1500 feet above the MDA’s HAA or HAT! It simply says “MDA” or “published Minimums”, both of which are based on MSL.

As always, all serious answers much appreciated...Thanks in advance

 

[Princedietrich]

If the airport has a circling approach that is available and authorized, then you take the HAA and add 1500 feet to get the required ceiling. Typically, most operators will be limited to a lowest circling of 450 which rounds up to 500 for the purposes of ceiling calculations so you're going to be in the near vicinity of 2000 which is the 91 requirement that we all know and love.

If the airport does not have a circling approach that is available and authorized, you take the HAA of the lowest approach (yes, that includes ILS) and add 1500. If this number is less than 2000, then you use 2000. If the number is more than 2000, you use that number. Again, you're going to be in the neighborhood of 2000 just like 91.

If you look at the plates (assuming you're using NOS like I do), you'll see three sets of numbers printed in the MDA boxes. The first is the MDA itself, expressed in MSL, and will be in bold text. The second is the AGL represented by that MDA, either as HAT for straight ins or HAA for circles, and will be in smaller text. The third is a published set of minimums for military use only, and will be in small text with parenthesis around it. The SECOND number, the small text with no parenthesis, is the one you will add the 1500 to.

 

[Bernoulli]

If the airport has a circling approach that is available and authorized, then you take the HAA and add 1500 feet to get the required ceiling. Typically, most operators will be limited to a lowest circling of 450 which rounds up to 500 for the purposes of ceiling calculations so you're going to be in the near vicinity of 2000 which is the 91 requirement that we all know and love.

If the airport does not have a circling approach that is available and authorized, you take the HAA of the lowest approach (yes, that includes ILS) and add 1500. If this number is less than 2000, then you use 2000. If the number is more than 2000, you use that number. Again, you're going to be in the neighborhood of 2000 just like 91.

Thanks for the reply...but how do you know you can use the HAA or the HAT? The reg does not say anything about HAA or HAT which are based on AGL. The reg states "above the lowest published MDA" or "Above the lowest published minimum" both of which are based on mean sea level...it does not say above the lowest MDA's HAA or HAT. I'm looking for proof here, and I'm not trying to split hairs because there is a big difference.

I can think of a very reasonable explanation for why the FAA may want you to do the calculation strickly based on MDA which is based on mean sea level as opposed to HAA or HAT which is based on AGL. Think about it. Using sticly MDA (based on mean sea level) the higher the airport's elevation the higher the cealing would need to be above the airport. Why would the FAA want to make the ceiling above the airport greater at higher elevation aitrports?...because higher elevation airport's weather changes way faster than lower elevation sea level airports simply because the temp dew point spread is closer and therefore the airport's weather can change much faster.

 

[minitour]

While we're on this reg: Something that's been bothering me.

So you've got an airport with 2 ILSs and 2 NDBs, one going each way. They publish straight in minimums for ILS, LOC and NDB and circling minimums for each approach.

Since these are not "circling approaches" (like a VOR-A, NDB-A, etc.) do you use the "height above circling MDA" or "height above lowest minimum or airport whichever is higher"?

:erm:

Sorry for the retardedness of this question...my brain cells this week are devoted to The Ohio State University
...

-mini

 

[Princedietrich]

The confusion here is a product of poor wording on the part of whoever wrote that particular reg. The Federales are unfortunately notorious for that.

When working with a circling MDA, whether it happens to be a circling approach like a VOR-A or circling minimums on an ILS, you are dealing with HAA. HAA is measured from the published field elevation for the airport. HAT on the other hand is measured from the touchdown zone of the runway in use. Since a circling approach isn't designed to put you down on a specific runway, it can't be measured off of the HAT for a specific runway. If you look in the front of the NOS plates in the section that explains how to read them, there will be a notation of how to figure HAT/HAA and which numbers represent which values on there.

For the purpose of finding a 135 alternate, you're looking for a circling minimum. It doesn't necessarily have to come from a circling approach like a VOR-A, and you can indeed use circling minimums from a LOC or ILS or straight in VOR or whatever.

 

[avbug]

The intent of the regulation is clear, if one will but exercise a fraction of the mental fortitude for which one's employer is paying. Splitting hairs or playing semantics doesn't change that. You need only convert AGL to ASL, or visa versa, to comply.

If you have a circling MDA which is HAT/HAA of 800', you need only add 1,500 to that to come up with an AGL altitude of 2,300'. If the airport is at sea level, or at 5,000 ASL/MSL, this doesn't change. It only requires using a little bit of one's brain.

14 CFR 1.1 defines ceiling as "...the height above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as "broken", "overcast", or "obscuration", and not classified as "thin" or "partial"."

Your are concerned about 135.223 which reads in part:

"(1) The ceiling will be at least 1,500 feet above the lowest circling approach MDA; or
(2) If a circling instrument approach is not authorized for the airport, the ceiling will be at least 1,500 feet above the lowest published minimum or 2,000 feet above the airport elevation, whichever is higher; and..."

Clearly the regulation is concerned with ceiling, which is an AGL altitude, and therefore you must do what is necessary to determine the AGL altitude from the various requirements applied to the regulation (such as MDA). This is not a particularly troublesome burden, nor rocket science. You need only take the information at hand, determine if the ceiling required to meet the requirements of 135.223(b) is higher than a 1,500' ceiling, and you're done.

Don't make it more difficult than it need be...and it is not, and need not.

 

[minitour]

For the purpose of finding a 135 alternate, you're looking for a circling minimum. It doesn't necessarily have to come from a circling approach like a VOR-A, and you can indeed use circling minimums from a LOC or ILS or straight in VOR or whatever.

Thanks.

-mini

 

[Bernoulli]

The intent of the regulation is clear, if one will but exercise a fraction of the mental fortitude for which one's employer is paying. Splitting hairs or playing semantics doesn't change that. You need only convert AGL to ASL, or visa versa, to comply.

If you have a circling MDA which is HAT/HAA of 800', you need only add 1,500 to that to come up with an AGL altitude of 2,300'. If the airport is at sea level, or at 5,000 ASL/MSL, this doesn't change. It only requires using a little bit of one's brain.

14 CFR 1.1 defines ceiling as "...the height above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as "broken", "overcast", or "obscuration", and not classified as "thin" or "partial"."

Your are concerned about 135.223 which reads in part:

"(1) The ceiling will be at least 1,500 feet above the lowest circling approach MDA; or
(2) If a circling instrument approach is not authorized for the airport, the ceiling will be at least 1,500 feet above the lowest published minimum or 2,000 feet above the airport elevation, whichever is higher; and..."

Clearly the regulation is concerned with ceiling, which is an AGL altitude, and therefore you must do what is necessary to determine the AGL altitude from the various requirements applied to the regulation (such as MDA). This is not a particularly troublesome burden, nor rocket science. You need only take the information at hand, determine if the ceiling required to meet the requirements of 135.223(b) is higher than a 1,500' ceiling, and you're done.

Don't make it more difficult than it need be...and it is not, and need not.

Thanks for the response Avbug...iI'm glad you chimmed in...I always learn from you. I agree that ceilings are always based on AGL and the way you interpret the reg makes complete sense to me...but why would the FAA not state "the lowest MDA's HAA or HAT", rather than saying "lowest MDA or published minimum" The way it's worded can be taken totally different from what you have posted. Do you know of any FAA legal interpretation on this topic?

 

[Bernoulli]

No one else has an opinion on this or anything to add?

 

[cvsfly]

Thanks for the reply...but how do you know you can use the HAA or the HAT? The reg does not say anything about HAA or HAT which are based on AGL. The reg states "above the lowest published MDA" or "Above the lowest published minimum" both of which are based on mean sea level...it does not say above the lowest MDA's HAA or HAT. I'm looking for proof here, and I'm not trying to split hairs because there is a big difference.

I can think of a very reasonable explanation for why the FAA may want you to do the calculation strickly based on MDA which is based on mean sea level as opposed to HAA or HAT which is based on AGL. Think about it. Using sticly MDA (based on mean sea level) the higher the airport's elevation the higher the cealing would need to be above the airport. Why would the FAA want to make the ceiling above the airport greater at higher elevation aitrports?...because higher elevation airport's weather changes way faster than lower elevation sea level airports simply because the temp dew point spread is closer and therefore the airport's weather can change much faster.

I agree that the FAA could write more clearly. But I think you are reading too much into it by virtue of the statemnet that the "MDA is based on MSL". It represents an AGL figure. Airport El. = 700', MDA is published as 1200 (500). 1200 is the MSL figure you level at with the altimeter, but it is 500 AGL. Use the AGL figure for these calculations.