Descending to 100' with ALS in sight

chuckles1225

chuckles1225

Well-Known Member
As far as questions go, they don't get much more picky or technical than this one. Had a student ask me it last week and I wasn't really sure what to tell him.

According to 91.175 a pilot is allowed to descend below their MDA/DA to 100 feet above touch down zone elevation once they have the approach lights in sight.

This little section is useful on those very low visibility ILSs when you break out just before or at your DA but only see the ALS, and you need that extra 100 foot descent to get the runway in sight.

My student's question was: do the approach lights actually need to be operable for this rule to apply? It would be very possible to see the physical MALSR or ALSF, etc. in the day even if the lights were off. I know its pretty out there and unlikely but I'm sure it has happened to pilots before. I told him it doesn't apply if the lights are inop. and you'd have to go missed just because that's the most conservative answer. Any thoughts?
 
I've always been told that you'd better be able to see the terminating bars if you're going to commit to land.

If you can't see those, I'd be gettin' outta dodge.

I think that'll cover most situations eh?
 
I've had this happen a few times. We get the infrastructure in sight before the runway (day time). The tower simply forgot to turn on the lights.

Here is the nail in the coffin of your question, as asked. What happens to your approach mins if the approach lighting system, or part of it, is INOP?
 
400A said:
Here is the nail in the coffin of your question, as asked. What happens to your approach mins if the approach lighting system, or part of it, is INOP?
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If I'm not mistaken, the required visibility goes up, but the DA is unchanged.
 
Xiph said:
If I'm not mistaken, the required visibility goes up, but the DA is unchanged.
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You are correct, the assumption on my part being if the vis is so bad that you are seeing the lighting structure but not the pavment, it is a fair bet that the increased vis required by the lights being OTS is far greater than the flight vis you currently have.
 
400A said:
You are correct, the assumption on my part being if the vis is so bad that you are seeing the lighting structure but not the pavment, it is a fair bet that the increased vis required by the lights being OTS is far greater than the flight vis you currently have.
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Very true
 
400A said:
Here is the nail in the coffin of your question, as asked. What happens to your approach mins if the approach lighting system, or part of it, is INOP?
Click to expand...
Xiph said:
If I'm not mistaken, the required visibility goes up, but the DA is unchanged.
Click to expand...
400A said:
You are correct, the assumption on my part being if the vis is so bad that you are seeing the lighting structure but not the pavment, it is a fair bet that the increased vis required by the lights being OTS is far greater than the flight vis you currently have.
Click to expand...


Ding Ding Ding...we have a winner. I hope I'm not alone in thinking that a student that would pose this question, gets it and is using his head. That was a great student question
 
You need an ALSF-1 or ALSF-2 approach lights to be able to take advantage of the 100ft descent rule. You must have the red terminating bars either on the end of the ALSF-1 or the red bars on the side of the ALSF-2 to come down to 100ft.

On another point, as far as required visibility. Part 135 (and I believe part 121) need to have the required visibility in order to start the approach (outside the FAF) but if they call it lower then you can still proceed with the approach. But what constitutes required visibility?

It's flight visibility (Part 91.175 and Part 1), so not what the tower see, or the AWOS, or the RVR, but what you see. As long as you can see 1800 or 2400 feet (or whatever) then you've just met that requirement.
 
kiloalpha said:
You need an ALSF-1 or ALSF-2 approach lights to be able to take advantage of the 100ft descent rule. You must have the red terminating bars either on the end of the ALSF-1 or the red bars on the side of the ALSF-2 to come down to 100ft.
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I thought that was to go below TDZE + 100?

The ALS (any configuration) in sight, you can go below DA (to TDZE+100')
The red terminating or side lights in sight allows you to go below TDZE+100

91.175 excerpt:
(i) The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable.
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SIUav8er said:
I thought that was to go below TDZE + 100?

The ALS (any configuration) in sight, you can go below DA (to TDZE+100')
The red terminating or side lights in sight allows you to go below TDZE+100

91.175 excerpt:
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:sitaware:

Failure on my part of properly deciphering FAR's! Good job!
 
Can't remember without looking it up, but some approach light systems stretch out 1/2 SM from the terminating bars don't they? Therefore would give you your "flight visibility"
 
kiloalpha said:
On another point, as far as required visibility. Part 135 (and I believe part 121) need to have the required visibility in order to start the approach (outside the FAF) but if they call it lower then you can still proceed with the approach. But what constitutes required visibility?

It's flight visibility (Part 91.175 and Part 1), so not what the tower see, or the AWOS, or the RVR, but what you see. As long as you can see 1800 or 2400 feet (or whatever) then you've just met that requirement.
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Sec. 135.225

IFR: Takeoff, approach and landing minimums.

(a) Except to the extent permitted by paragraph (b) of this section, no pilot may begin an instrument approach procedure to an airport unless--
(1) That airport has a weather reporting facility operated by the U.S. National Weather Service, a source approved by U.S. National Weather Service, or a source approved by the Administrator; and
(2) The latest weather report issued by that weather reporting facility indicates that weather conditions are at or above the authorized IFR landing minimums for that airport.
(b) A pilot conducting an eligible on-demand operation may begin an instrument approach procedure to an airport that does not have a weather reporting facility operated by the U.S. National Weather Service, a source approved by the U.S. National Weather Service, or a source approved by the Administrator if--
(1) The alternate airport has a weather reporting facility operated by the U.S. National Weather Service, a source approved by the U.S. National Weather Service, or a source approved by the Administrator; and
(2) The latest weather report issued by the weather reporting facility includes a current local altimeter setting for the destination airport. If no local altimeter setting for the destination airport is available, the pilot may use the current altimeter setting provided by the facility designated on the approach chart for the destination airport.
[(c) If a pilot has begun the final approach segment of an instrument approach to an airport under paragraph (b) of this section, and the pilot receives a later weather report indicating that conditions have worsened to below the minimum requirements, then the pilot may continue the approach only if the requirements of Sec. 91.175(l) of this chapter, or both of the following conditions, are met--.....
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It is not flight visibility that determines whether or not we can begin an approach. The ground facility must report at or above minimums or we don't even start the approach.

edit to add:
You might be thinking of the requirment to have necessary flight visibility prior to descending below DH/MDA and for landing.
 
SteveC said:
It is not flight visibility that determines whether or not we can begin an approach. The ground facility must report at or above minimums or we don't even start the approach.

edit to add:
You might be thinking of the requirment to have necessary flight visibility prior to descending below DH/MDA and for landing.
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Steve, you're right, I was thinking of a ground facility calling out the required visibility (or better) prior to reaching the FAF on an approach

However once you pass the FAF, what YOU see is what dictates the ability to land and descend below DA/MDA.
 
IF you have an ALSF-I or ALSF-II and you have the 'red siderow or red terminating bar' in sight and distinctly identifiable, you do not have to stop at 100 HAT. You are allowed to continue to land, provided you determine to have the required visibility for the approach, and you can make normal maneuvers and rates of descent.

Huge misconception on this FAR by students and instructors alike.

ALS in sight, that do not contain red siderow or red terminating bars, you continue the descent to 100HAT.
 
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