Going Below Decison Height

[AmexFlyer]

Question for all you guys.....At any time, are you allowed to descend below the Decision Height as you're executing the Missed Approach Procedure? 91.175 has been read and it doesn't specify whether you can descend below DH during missed approach. Had a good debate with other CFI's.

 

[bigD]

If I'm on the glideslope and at the DH without the runway in sight - I can't avoid dropping lower without modifying the laws of physics that prevent me from instantaneously changing a ~500fpm descent into a climb.

Do a search on this board for this question. I remember it being rehashed many times over the years.

 

[flx757]

My answer is yes. Decision Height (or Altitude) is where you make the decision to execute the missed approach or continue if the required visual cues are present. By making the decision at DH(A) to execute the missed approach, especially in a large aircraft where mass and inertia are a greater factor, you will invariably descend below the DH(A) during the missed approach maneuver. In order to prevent this, you would have to begin the missed approach at some altitude before DH(A), and no where have I ever heard or read that this is required.

 

[cookmg]

I agree with the other posters. The definition of DH is the height at which a decision must be made to either continue the approach or to execute a missed. This contrasts to MDA which is defined as the lowest altitude to which descent is authorized.

I think the definitions per 14 CFR 1.1 explain why it's legal to descend below DH. I also think that the practical explanations given by the other posters show why it makes sense in real-world flying.

 

[Cornelius]

WHAT, you always have the lights in sight no matter what. Why brief the Missed? J/K!!

 

[80/20]

Agree with the rest - it's normal to go below DH during a missed approach from minima.
TERPS actually assume that you will go a bit below DH. As Cookmg says, this contrasts to MDA - the lowest that is authorized.

A bit off the topic but hopefully relevant:
Flying non-precision approaches with a constant angle (CANPA) is a very god alternative to the old step down technique. But you will bust MDA if you fly CANPA all the way down to MDA and go missed. CANAPA is being used more and more and some airlines have increased CANPA MDA with a 50' buffer on their customized charts. FAA suggests to use an approved, standard MDA buffer altitude or other procedures to avoid descents below MDA during missed approaches.

 

[WrightAvia]

Lights in sight'll get you a NEW DH....

Yea...make your decision AT Decision Height. Go around if you don't have 91.175 met at DH. If you get the lights you can come down to your new Decision Height. 100 Feet above touch down zone elevation. If it wasn't for this allowance in the 91.175, you'd see alot more holding and wasted flying.

I think the reason you see more debate about this topic amongst instructors, is the fact that their instructors never had any real life experience in IFR (this is not a knock on low time instructors..we all were there at one time and that is just the way this system works). I ran into the same problem at the 141 school I went to also. The blind leading the blind. Eventually, I taught at a 10 day instrument course place and learned to teach and understand 91.175, but never had any real hands on practice at it, till I started flying 135.

Now it happens to me, if not on a weely basis, many times a day (and night). Flying an ILS with VIS at a 1/2 mile and Vertical Visibility 100. You'll make out the LIGHTS at your 200 foot DH, reset the radar altimeter to 100 and continue on down till you reach your new Decision Height of 100 feet TDZE. (don't forget to note what this NEW altitude will be on your altimeter, also).

 

[Singlecoil]

Re: Lights in sight'll get you a NEW DH....

You'll make out the LIGHTS at your 200 foot DH, reset the radar altimeter to 100 and continue on down till you reach your new Decision Height of 100 feet TDZE. (don't forget to note what this NEW altitude will be on your altimeter, also).

I just want to make sure this doesn't confuse some folks. The barometric altimeter is always primary when shooting a cat I ILS. Radar altimeter in this case is nothing more than a backup, and frankly isn't worth much. Try shooting a Cat I to 16R at KSEA with the radar altimeter, and you will hit the ground. There is a ravine right in the area of the middle marker that makes the radar altimeter lie to you.

If you are strictly flying in an area like Florida, then it won't be a big deal, but come out west or go to Alaska and it will be a different story. Maybe it would be better to learn and practice the proper way from early on.

 

[AA717driver]

"Reset the radar altimeter"?

At 200' they are telling people to reset the radar alt. to 100'? Well, at least we know what the cause of the crash will be.

At 200' if you see the lights, land. If you lose the lights below DH, go around.

I was taught by TWA instructors(just my background, you may have been taught differnently) that if you lose the lights below DH, you go around. This was done routinely in the sim. and it was also taught that you might even feel the wheels touch briefly--but you are still committed to a go-around.TC

 

[flx757]

I have to agree with singlecoil and AA717 on this deal. RA has no place in CAT I approaches, other than as a backup reference, if at all. There is no "new DH" at 100' above TDZE. The DH(A) remains what it was to begin with, and if at any time below DH(A) you lose the required visual reference, you execute a missed approach.

 

[cookmg]

You don't get a new decision height after you have the ALS in view. But, I think wrightavia was pointing out that the ALS is only only good enough to get you to 100ft agl unless the red row or terminating bars come into view, or one of the other required visual refrences comes into view. Like the others said, this is not a new decision height, because if at any time below 200agl you lose the required visual refrences for that altitude it is a missed.

 

[80/20]

Agree with singlecoil, flx757 and AA717, RA minima is for low visibility procedures only - not for CAT I. When landing in marginal conditions, my personal focus will be given to tasks other than RA adjustments because the seconds from DH to touchdown goes fast.

 

[WrightAvia]

ok...the use of the phrase NEW decision

HT was wrong.

As far as what I said about the radar altimeter... Shut up. I never said it was primary. I never said it was secondary. I just said for me, it was set and and then reset again at 100 FEET! I fly single pilot IFR and even I got time for turning a knob...IT'S A TWIST OF A KNOB...whoooooboy!??!. Sorry if I use ALL the tools at my disposal in my cockpit. No law against it. I spose next you're going to tell me not to waste time setting up the second nav for the ILS, cause there is no need, nor requirement, in backing up the PRIMARY source of NAV on the approach with a Secondary. Not to mention, that means ALL THREE OF MY ILS HEADS ARE IN USE AND DISPLAYING RELEVENT INFORMATION. Nobody told you that you had to use 3 ILS heads your instrument training either! I spose you're going to tell me it's wrong to set my KLN-89 ON the FAF and OBS align the moving map to the final approach course...so I can monitor my progress on the approach? Next your going to say, using the altitude preselect alarm is a waste of time and redundant, cause the BARO altimeter is primary. Why bother setting the co-pilot's altimeter or DG for that matter.

Also as far as the comment on alaska or mountainous flying, If you are flying somewhere where the ILS eventually gets shot to 100 feet of TDZE and is so close to mountainous terrain, that a mountain or other obstacle is actually protruding up through the landing lights as a collision hazard...than that IS YOUR FREAKING PROBLEM, not mine. I can't imagine any airport where they put the landing lights for an ILS down with obstacles sticking up out of them. The middle marker on most approaches are on average .4 to .5 or .6 miles from the threshold and the LENGTH OF ALSF-I and ALSF-II lighting arrays is at least 2,400 to 3,000 feet long! Even MALSR and SSALR lighting is at least that long. That means when you hit your 200 foot DH you are almost at the end of the landing light system and at 100 FEET ABOVE TDZE, you are almost half way up the lighting system. If you got a mountain or other obstacle sticking up in them lights...tough, it was YOUR approach briefing MR, not mine!

I'm sorry guys, I don't have some second pilot riding along...doing the two pilots flying calls song and dance routine. I have to do it ALL on my own...I set and identify everything, brief the approach plate hard and fly it, just like it was briefed. I know the limitations of the radar altimeter. I know what's primary for altitude in an instrument airplane and I know they aint got no mountains in no approach lighting system. The stupid canal on the way into one of my ILS's plays that same stupid terrain game with my radar altimeter...duhhhhh.

I ALSO KNOW THAT IN REFERENCE TO THE ORIGINAL QUESTION, AT DH, YOU CAN MAKE A DECISION TO GO AROUND AND GO BELOW DH FOR THE TRANSITION. I ALSO KNOW THAT AS CAPTAIN OF MY SHIP, HOWEVER SMALL AND INSIGNIFICANT THAT MAY SEEM TO YOU, THAT ANY TIME I AM ABOVE GROUND...I AM AT A DECISION HEIGHT.

 

[WrightAvia]

80/20 with all due respect...

at 500 feet per minute descent rate...lets just say my scan is good enough to use the radar altimeter in conjunction with all the other goodies in my plane. As I reach DH, IF I got the lights...I reset the radar altimeter for 100 feet, cause I have to be HEADS UP at the same time I'm monitoring the Glideslope and localizer, the rest of the plane, the auto pilot, the two barometric altimeters, the missed approach plan, balancing my check book and finishing a flight log. I think if I want to use my 12 seconds from DH, to 100 feet above TDZE to turn a knob so I have one last alarm to keep me from drilling the tarmac between the touchdown zone markings on the runway...then that is my choice. It's 12 seconds from DH, to 100 feet above TDZE. 12 seconds.

I really think some of you high time, heavy airplane guys, have lost a certain depth perception when It comes to single pilot IFR in commercial application. That is no bad reflection on you, but I got work to do and I use all the tools available to me to get the job done. I can use my 12 seconds to set a wake up alarm, instead of hitting the snooze button.

 

[Singlecoil]

Re: ok...the use of the phrase NEW decision

Also as far as the comment on alaska or mountainous flying, If you are flying somewhere where the ILS eventually gets shot to 100 feet of TDZE and is so close to mountainous terrain, that a mountain or other obstacle is actually protruding up through the landing lights as a collision hazard...than that IS YOUR FREAKING PROBLEM, not mine. I can't imagine any airport where they put the landing lights for an ILS down with obstacles sticking up out of them. The middle marker on most approaches are on average .4 to .5 or .6 miles from the threshold and the LENGTH OF ALSF-I and ALSF-II lighting arrays is at least 2,400 to 3,000 feet long! Even MALSR and SSALR lighting is at least that long. That means when you hit your 200 foot DH you are almost at the end of the landing light system and at 100 FEET ABOVE TDZE, you are almost half way up the lighting system. If you got a mountain or other obstacle sticking up in them lights...tough, it was YOUR approach briefing MR, not mine!

Why you are getting defensive about this, I have no idea. If I see a post on this board that I think is misleading or perhaps harmful to folks learning to fly, I'm going to comment.
I guess you don't know what the word ravine means. It is the opposite of a mountain. It is a dip. In other words, when you fly over it, your radar altimeter says you are higher than your actual altitude above TDZE.
Here is the approach to Seattle I was talking about. Note how the approach lights are built right over the ravine that the cars are driving on.

KSEA 16R

Your original remark of "don't forget to note what this NEW altitude will be on your altimeter, also" leaves the impression that this is not terribly important. The truth is the baro altimeter is the only accurate source for how high you are above TDZE.

Flame away, I know 80/20, AA717Driver, and FLX757 got my back.

 

[WrightAvia]

I looked at that picture....

and I still see enough turf to get a 100 foot alert on a Radar Altimeter. I see that you got homies that got your back. I see that I made a big mistake on the "new dh" 91.175 operation below DH thing. I see I'm still going to include my radar altimeter in my instrument brief and scan, to alert me when I have 12 seconds or maybe less left before IMPACTING the zone... because only I, got MY back. I also see that DH was meant to be flown below, legally. But then WE all knew that before this thread...except for the original poster. But he knows that now.

 

[80/20]

This thread is getting interesting. Different opinions, various types of operation and background from people who like what we do and have practical experience. That's what I learn from and why I like our forum!

I think we have clarified how to interpret the regulations and required procedures. We are now discussing various techniques. Keep in mind that a technique is one way, but not the only way to accomplish a task. WrightAvia has a good point when he want's to use all available resources. It is not prohibited to include the RA in the scan, it is actually recommended by many to use it; one example is for call-outs not related to minima during all approaches.

We have limited workload capability. How much we can handle varies from person to person and to a great extent with experience. When training new crew I would initially prioritize required items.

I have always had the greatest respect for single pilot IFR and consider this to be very demanding.

Let's appreciate our different opinions and professional passion - think how boring it would be if we were all shaped by the same mold.

 

[Checks]

I have flown single pilot IFR the last couple of years and I personally dont see the advantage of reseting the altimeter at DA if you see the lights. Personally, I am locked into my scan at DA and if the ALS comes into sight peripherally I will just go down 100 additional feet on the altimeter(assuming a 200ft ILS). I am not good enough of a pilot to add knob twisting and additional instruments into my scan when that close to the ground. I believe in making things as simple as possible.