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Home > Flying the Plane > When is a pilot allowed to descend below the decision altitude?
When is a pilot allowed to descend below the decision altitude?
Flying - Flying the Plane
Monday, 26 February 2007 20:18

Hello Captain Lim,

Let me begin by saying that your website is absolutely wonderful and I*ve been entertained the last hour and a half reading your postings. Aviation is an interesting subject to study and you are so fortunate to be the pilot of a Boeing 777. Its every private pilots* dream to some day, fly the heavies!

I*m an Air Force weather forecaster stationed at Elmendorf AFB in Anchorage, Alaska. Part of my job*s responsibility is to forecast weather conditions and provide verbal weather briefings to military pilots. Most of the pilots I work with are Army pilots stationed at Fort Richardson. It*s interesting how military pilots follow a different set of rules compared to commercial pilots. For example, Army pilots are allowed to shoot the approach when the airfield is below the minima. I don*t believe Air Force or commercial pilots are authorized to do that. But anyway, I have a question that*s been on my mind for quite some time and hopefully you*ll have the answer.

As you probably know from experience, weather in Alaska can be very harsh especially along the Bering Coast and Aleutian chain. I do not understand how certain pilots manage to land in unfavorable conditions when the weatherman is observing ceilings below approach category "A".

Lynden Air Cargo, Phoenix Air, and Northern Air Cargo regularly fly to Shemya (Eareckson AS) on the Western Aleutians, Tin City LRRS, Cape Romanzof LRRS, Sparrevohn LRRS, etc. Lynden Air Cargo operates an L100 (C-130) aircraft that falls under a CAT II approach category. When the L100 is scheduled for a flight, our office is required to issue a terminal aerodrome forecast (TAF) and a weather observer is required to take observations at the location 5 hours prior to aircraft departure ...or 8 hours if the AWOS/ASOS is not operational.

A couple of weeks ago, Lynden Air Cargo scheduled a flight to Cape Newenham that is on the Bering Coast. The CAT C airfield approach at Cape Newenham, AK is 1119-3. The CAT A approach is 1119-2 3/4, not much of a difference. Circling is not authorized. The weather observer had been reporting 500-800 foot ceilings all morning with 1 to 2 miles visibility that is well below the airfield minima. Well, the weather observer called to notify me the aircraft made it in and they were closing weather. Hmmmmm...

I do not understand how the pilots manage to land an aircraft under those conditions. Are there any circumstances when a pilot is allowed to descend below the decision altitude? I*ve seen this happen many times and in some ways I think maybe the pilots bend the rules a bit so the company can deliver the cargo. Or perhaps they have special authorization? After all, if they don*t deliver the cargo they don*t make money. Flights even land at Shemya in OVC001 1/4SM on a standard ILS (200 1/2)? Not to mention winds 35G45kts.

From what I understand, Phoenix Air can go 100 feet below the CAT A approach at Shemya. Have you every heard of an exception like that? Based on your experience as a pilot, do you have any explanation on how aircraft manage to land in unfavorable weather conditions like the way aircraft land at Cape Newenham and other remote sites?

Also, off the subject, I*ve seen a Korean Air 747 land at Elmendorf when the destination was supposed to be Anchorage. Anchorage has a CAT III ILS so I*m not sure why this happened. Do you have any ideas?

Your time is greatly appreciated, and once again keep up the good work on your web page.

Thanks in advance!

Regards,

Bryan Cribb

Hi Bryan,


As a weather forecaster, I understand why you are concerned about how some military and commercial pilots are able to land in weather that you have forecasted to be below the minima for a safe landing. You are right about the minimas - pilots, regardless of whether they are from the Air Force or commercial airlines, shall not fly below the specified minimum altitudes of any runways unless the conditions are met. For instance, if I were to perform a CAT III A auto landing, I must see at least 3 center line lights or the runway marking by the decision altitude of 50 feet above the ground level. If not, I would carry out a go around.


I know of no circumstances whereby a pilot is allowed to descend below the decision altitude (minima) unless the landing conditions are met. Perhaps what may be unfavorable to you as a forecaster at the time of your report may be different from the actual weather when the pilot flew the approach. He could see, for example, the 3-center line lights from the cockpit on the CAT III A approach and made the decision to land at 50 feet. But on a CAT I Approach, he must have a ceiling of 200, not 100 feet and a visibility of 550 meters (1800 feet), not ? statute miles. Winds gusting from 35 to 45 knots at Shemya is not unfavorable per se unless they were crosswinds! (Yes, any pilots who bend the rules do so at their peril!)

Do I have any explanation on how some aircraft manage to land in unfavorable weather conditions? I can only say that most airplanes can get into runway with poor visibility by use of the auto landing system provided the requirements
are satisfied.

I am not sure why the Korean Air Boeing 747 landed at Elmendorf instead of Anchorage even though the latter has a CAT III ILS. Perhaps Elmendorf weather was better and that of Anchorage was marginal or the 747 auto landing equipment was incapable of performing at CAT III ILS approach at that time.

Happy Forecasting!

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ATP
You can descent below DA when executing a missed approach, therefore your statement of not knowing of ANY time it's allowed is not valid.
John , 06 Aug, 2010

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