Can we descend below DA on the go around due to coupled height loss ?
Let'sgoflying!
Touchdown! Greaser!
If the regs on height loss at DA/DH differ from handflown vs coupled, I missed that.
In CAT I approaches, I have read that each aircraft’s autopilot has a minimum altitude below which it must be disconnected.
(CAT II/III äre always coupled to the ground, I believe)
Height loss is allowed/expected when a missed is initiated on a precision approach.
Height loss is not, when going missed on a NPA.
“During the transition to a climb in a missed approach procedure out of an ILS or LPV approach (left), the airplane is allowed to dip slightly below the decision altitude or decision height. Flying an LNAV, LNAV/VNAV, or other nonprecision approach (right), a pilot must hold the minimum descent altitude until reaching the missed approach point.”
https://www.aopa.org/news-and-media/all-news/2017/march/pilot/on-instruments-making-the-miss
In CAT I approaches, I have read that each aircraft’s autopilot has a minimum altitude below which it must be disconnected.
(CAT II/III äre always coupled to the ground, I believe)
Height loss is allowed/expected when a missed is initiated on a precision approach.
Height loss is not, when going missed on a NPA.
“During the transition to a climb in a missed approach procedure out of an ILS or LPV approach (left), the airplane is allowed to dip slightly below the decision altitude or decision height. Flying an LNAV, LNAV/VNAV, or other nonprecision approach (right), a pilot must hold the minimum descent altitude until reaching the missed approach point.”
https://www.aopa.org/news-and-media/all-news/2017/march/pilot/on-instruments-making-the-miss
tsts4
Pattern Altitude
What rule says that? To the best of my knowledge it’s not part 91. Having to go missed below DA/DH because you lose visual ref to the runway environment is one thing, but to say that gives you authority to dip below DA/DH because you were slow to arrest your decent either while hand flying or via automation is something else IMO.That said I also don’t think it’s the end of the world but would like to know the actual reference for my own education in order differentiate between what actually happens in everyday flying vs the letter of the law.
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Mongoose Aviator
Line Up and Wait
Pinecone
En-Route
Any think about it. If on a non-precision approach, your flying straight and level at MDA, so no reason to dip below.
On a precision approach, you are descending, and the airplane has inertia, so if you fly all the way to DH, it is impossible to not dip below a bit. The airplane does not react instantly
On a precision approach, you are descending, and the airplane has inertia, so if you fly all the way to DH, it is impossible to not dip below a bit. The airplane does not react instantly
RussR
En-Route
Absolutely some height loss is expected and anticipated. You will not find anywhere it written that you can't descend below DA in the process of going missed - because it's a "DECISION altitude", not a "Minimum Descent Altitude" - it's in the term itself. Even the Instrument ACS acknowledges this through different wording of the standards for a non-precision and precision approach.
It's even accounted for in the procedure design, a normal 3 degree glideslope allows for up to about 76 feet of height loss, as it is acknowledged that 1) it takes reaction time once that decision is made to go missed, and 2) airplanes have inertia.
This doesn't mean that you can just make a really slow decision to get another 100 feet lower on the approach, but anybody who is teaching that DA is a "minimum altitude" that must not be violated is incorrect.
It's even accounted for in the procedure design, a normal 3 degree glideslope allows for up to about 76 feet of height loss, as it is acknowledged that 1) it takes reaction time once that decision is made to go missed, and 2) airplanes have inertia.
This doesn't mean that you can just make a really slow decision to get another 100 feet lower on the approach, but anybody who is teaching that DA is a "minimum altitude" that must not be violated is incorrect.
luvflyin
Touchdown! Greaser!
91.175 (c)
(c) Operation below DA/DH or MDA. Except as provided in § 91.176 of this chapter, where a DA/DH or MDA is applicable, no pilot may operate an aircraft, except a military aircraft of the United States, below the authorized MDA or continue an approach below the authorized DA/DH
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Kayoh@190
Most of the commercial world has gone away from 'diving and driving' on non-precision approaches, so the typical procedure is to add 50 feet to any MDA and call that our Derived Decision Altitude, or DDA. As the name implies we treat the DDA as a decision altitude, and the 50 feet gives us some buffer for inertia to carry us through the DDA, but not below the actual MDA.
tsts4
Pattern Altitude
I get that, hence my question as to where does it say you can dip below the DA/DH per the AoOA article you posted the link to. I get that as a practical matter going below the DA/DH is going to happen, but where’s the rule that actually says it technically ok?
Mongoose Aviator
Line Up and Wait
That was just posted. Could you clarify exactly what you are asking and the reason? Potentially that could help with any replies so you do not just keep getting the same information which appears not to be precisely hitting your concern.
luvflyin
Touchdown! Greaser!
Yeah. It doesn’t explicitly say you cannot descend below DA/DH. But it allows it by not saying you cannot do it. It says you may not descend below MDA. It does not say that about DA/DH. It just says you may not continue the Approach.
tsts4
Pattern Altitude
My interpretation of 91.175 is it doesn’t give any leeway for “dipping” below the DA/DH when going missed. The link to the AOPA article stated the rules allowed it. I’m just curious as to what specific rule?
Clip4
Final Approach
What is the FA?
luvflyin
Touchdown! Greaser!
What about the autopilot. Is this accounted for in the TSO? That must be able to respond within those limits? STC? Do some of those say you can’t use this for coupled approaches. Some approaches say ‘coupled approaches N/A.’ Does this scenario factor into that sometimes?
luvflyin
Touchdown! Greaser!
91.175 (c). It does give leeway for ‘dipping’ by not saying you cannot descend below like it does for MDA.
MauleSkinner
Touchdown! Greaser!
There’s a difference in the wording that makes it true…
“no pilot may operate an aircraft…below the authorized MDA” means no leeway, you cannot go below MDA unless you can see what you need to continue for landing.
“continue an approach below the authorized DA/DH” means you can’t continue the approach, but you can operate briefly below DA while initiating the missed approach.
MauleSkinner
Touchdown! Greaser!
Your AFM Supplement for the autopilot will tell you what you can and can’t do, as well as how to properly operate the autopilot to stay within design parameters.
Larry in TN
En-Route
From the AIM, 5-4-5:
Chart Terminology.
Chart Terminology.
- Decision Altitude (DA) replaces the familiar term Decision Height (DH). DA conforms to the international convention where altitudes relate to MSL and heights relate to AGL. DA will eventually be published for other types of instrument approach procedures with vertical guidance, as well. DA indicates to the pilot that the published descent profile is flown to the DA (MSL), where a missed approach will be initiated if visual references for landing are not established. Obstacle clearance is provided to allow a momentary descent below DA while transitioning from the final approach to the missed approach. The aircraft is expected to follow the missed instructions while continuing along the published final approach course to at least the published runway threshold waypoint or MAP (if not at the threshold) before executing any turns.
RussR
En-Route
I can't speak to the TSO/STC questions.
As far as "autopilot coupled approach NA below XXX MSL" remark on some approaches, this is due to Flight Inspection finding an unacceptable amount of what's called "rate of change/reversal" of the glideslope. There is some padding in this altitude, but I don't think it's related to height loss (since at that point you'd be continuing manually). Basically, the altitude in the note is 50 feet higher than the point at which the out-of-tolerance condition is determined.
flyingcheesehead
Touchdown! Greaser!
MauleSkinner
Touchdown! Greaser!
You meant to type D anf F, but you dat dingeref it, fifn’t you?I'm guessing that it's a typo, given that the F and the D are afjacent on the keyboarf.
Pinecone
En-Route
The key is, when you start your go around, you are no longer continuing the approach, you are flying the missed approach.
luvflyin
Touchdown! Greaser!
Yup.
John Collins
En-Route
The relevant portion of the rule says "continue an approach below the authorized DA/DH". At the DA/DH you make the decision. Below the DA/DH if your decision was to miss the approach, you immediately begin arresting the descent and transitioning to a missed approach climb by pitching up and adding power. If the FAA did not allow any descent below the DA/DH the rule would have said so. They used the word continue, meaning you have to start the process of missing the approach and can't continue on the original descent path to the runway. The FAA would not have needed to make a distinction in the relevant wording:
no pilot may operate an aircraft below the authorized MDA or continue an approach below the authorized DA/DH unless—
and the regulation could have simplified it to read as:no pilot may operate an aircraft below the authorized MDA or DA/DH unless—
Clearly that would be impossible. Those words have meaning.
Sluggo63
Pattern Altitude
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Sluggo63
And this always isn't true either.
At my place, there's a provision to use MDA as DA under certain criteria. That means that we can fly an RNAV approach as a constant descent and use the MDA as a DA and dip below it as the decision is made at the MDA. If we don't meet the criteria to use MDA as DA, then we do have to add the 50 feet and make it a DDA.
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Kayoh@190
We had a few provisions for a long time as well, but a couple of years ago they were removed and now all charted MDAs require the additional 50 feet. Not sure what happened.
Larry in TN
En-Route
Yep. Same here.
Sluggo63
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Sluggo63
Huh. Interesting... I just had to check my FOM to make sure it wasn't taken out and I missed it (which is not beyond the realm of possibility).
We still have it in there.
MauleSkinner
Touchdown! Greaser!
I see quite a few that have “MDA as a DA” in their OpSpecs with no DDA requirements.Huh. Interesting... I just had to check my FOM to make sure it wasn't taken out and I missed it (which is not beyond the realm of possibility).
We still have it in there.
unfortunately a lot of the pilots can’t come up with what approach/runway criteria have to exist in order for them to take advantage of the OpSpec.
John Collins
En-Route
Huh. Interesting... I just had to check my FOM to make sure it wasn't taken out and I missed it (which is not beyond the realm of possibility).
We still have it in there.
This can apply to a certified operator such as an airline. The rational is that they fly only into specific airports that have an NPA to a runway that is also served with a vertically guided approach procedure such as an ILS, LPV, or LNAV/VNAV, meaning that the approach path to the runway is clear of obstacles, not by virtue of the NPA, but other procedures. The OpSpec lists the criteria that the runway must satisfy to use a MDA as a DA. There are three main ones.
1) Serves a runway that has a published RNAV IAP (“RNAV (GPS),” “RNAV (RNP),” or “GPS” in the title) with a published LNAV/VNAV or RNP DA and:
a) Is selected from an approved and current database.
b) Has the exact published final approach course as the RNAV IAP.
c) Has a published VDA coincident with or higher than the barometric vertical guidance GS on the published RNAV IAP.
2) Serves a runway that has a published ILS, GLS, or RNAV IAP with LPV minima and:
a) Is selected from an approved and current database.
b) Has the exact published final approach course as the ILS, GLS, or RNAV IAP.
c) Has a published VDA or GS coincident with or higher than the GS on the published ILS, GLS, or RNAV IAP.
3) Serves a runway to an airport operating under 14 CFR part 139 with a VGSI.
a) The VDA or GS on the published final approach course must be coincident with or higher than the published VGSI descent angle.
b) The published final approach course is within plus or minus 4 degrees of the runway centerline (RCL) course.
For part 91, you do the DDA thing.a) Is selected from an approved and current database.
b) Has the exact published final approach course as the RNAV IAP.
c) Has a published VDA coincident with or higher than the barometric vertical guidance GS on the published RNAV IAP.
2) Serves a runway that has a published ILS, GLS, or RNAV IAP with LPV minima and:
a) Is selected from an approved and current database.
b) Has the exact published final approach course as the ILS, GLS, or RNAV IAP.
c) Has a published VDA or GS coincident with or higher than the GS on the published ILS, GLS, or RNAV IAP.
3) Serves a runway to an airport operating under 14 CFR part 139 with a VGSI.
a) The VDA or GS on the published final approach course must be coincident with or higher than the published VGSI descent angle.
b) The published final approach course is within plus or minus 4 degrees of the runway centerline (RCL) course.
MauleSkinner
Touchdown! Greaser!
It can also apply to 135 operators who can fly into random airports that may not have other approaches that the authorization is based on. In which case, a deeper understanding of the requirements is necessary.
Mongoose Aviator
Line Up and Wait
@John Collins - In another post you had stated that MDA requires minimum of 250' clear of all obstacles to the MAP. Does that apply equally to DA as well MDA? Does that then imply that any DA/MDA which is 250' AGL or lower has a clear path to the runway threshold crossing height (TCH) free of all obstacles? Are all ILS approaches always clear of all obstacles on the glideslope from the FAF to the runway TCH?
A post of yours I read mentioned the "stipple" (I call it the little grey shaded course pointer going all the way to the runway threshold - your term is so much shorter). For RNAV approaches, is the presence of the stipple the best guarantee of obstacle free glidepath all the way to the runway TCH for RNAV approaches? I see LPV w/DA (and various RNAV MDA) approaches where some have the stipple and some do not have the stipple.
Although I respect the viewpoint of not descending below the MDA without the runway environment in sight, I also appreciate being able to learn from the approach plate if the glidepath is free of obstacles.
John Collins
En-Route
An RNAV NPA uses a MDA which is based on a level surface with a 250 foot OCS between the FAF nd the MAP. The highest obstacle may be anywhere along the FAC. A vertically guided LPV or ILS approach uses a sloped OCS, so the closer you get to the runway threshold, obstacles must be lower.
Simplified, the visual segment extends vertically from the VDP or DA height to a point 200 feet before the threshold at the threshold elevation. Straight in approaches evaluate two sloped surfaces in the visual segment, a 34:1 and a 20:1. If the 34:1 visual segment is not penetrated by any obstacles, the FAA will chart the stipple. If the 20:1 surface is not penetrated, a VDP will be charted for a NPA procedure. If neither of these slopes is clear, then the procedure will not show either a stipple or a VDP and the procedure will be NA at night unless mitigated by a VGSI. If flight check determines that obstacles in the visual segment are present that require the aircraft to maneuver to avoid them because following the VDA is too close to the obstacle, then the procedure will have the note: Obstacles-Visual Segment. For an ILS or LPV, the 34:1 must be clear in order to support a visibility < 3/4 mile and the 20:1 must be clear to support a visibility < 1 mile. The KONO RNAV (GPS) Rwy 15 is an example where an LPV is published, yet the visual segment is not clear on either a 34:1 or 20:1. There is another surface that is narrower than the visual segment surfaces that must be clear in order for an approach with vertical guidance to be authorized. It is termed the GQS (Glidepath Qualification Surface) and is intended to provide a descent path from DA to landing free of obstacles that might cause the aircraft to destabilize the descent. The point always is that when you are below the MDA or DA, you must be able to avoid any obstacles by visually seeing and avoiding them.
Simplified, the visual segment extends vertically from the VDP or DA height to a point 200 feet before the threshold at the threshold elevation. Straight in approaches evaluate two sloped surfaces in the visual segment, a 34:1 and a 20:1. If the 34:1 visual segment is not penetrated by any obstacles, the FAA will chart the stipple. If the 20:1 surface is not penetrated, a VDP will be charted for a NPA procedure. If neither of these slopes is clear, then the procedure will not show either a stipple or a VDP and the procedure will be NA at night unless mitigated by a VGSI. If flight check determines that obstacles in the visual segment are present that require the aircraft to maneuver to avoid them because following the VDA is too close to the obstacle, then the procedure will have the note: Obstacles-Visual Segment. For an ILS or LPV, the 34:1 must be clear in order to support a visibility < 3/4 mile and the 20:1 must be clear to support a visibility < 1 mile. The KONO RNAV (GPS) Rwy 15 is an example where an LPV is published, yet the visual segment is not clear on either a 34:1 or 20:1. There is another surface that is narrower than the visual segment surfaces that must be clear in order for an approach with vertical guidance to be authorized. It is termed the GQS (Glidepath Qualification Surface) and is intended to provide a descent path from DA to landing free of obstacles that might cause the aircraft to destabilize the descent. The point always is that when you are below the MDA or DA, you must be able to avoid any obstacles by visually seeing and avoiding them.
Mongoose Aviator
Line Up and Wait
Thanks @John Collins !!!
My home airport's RNAV approach does not have a VGSI and doesn't have either the stipple or VDP charted. But it isn't NA at night. So there's a charting error?
John Collins
En-Route
If the obstruction is lit that penetrates the 20:1 slope, then the NA at night is not required. Also the GQS surface is clear because the procedure has a vertically guided LPV option. PS, you could have made it easier to look up the procedure by naming it, RNAV (GPS) Rwy 18 3MY rather than making me look up the fixes shown and figure out where it was.
Didn't mean to be cryptic, just thought the picture told the story needed.If the obstruction is lit that penetrates the 20:1 slope, then the NA at night is not required. Also the GQS surface is clear because the procedure has a vertically guided LPV option. PS, you could have made it easier to look up the procedure by naming it, RNAV (GPS) Rwy 18 3MY rather than making me look up the fixes shown and figure out where it was.
John Collins
En-Route
I do a lot of support calls. Only rarely is there sufficient information provided to analyze the request. Most are worse than yours: "My GPS is broken, what's up? " To understand if it might be a charting error, I start by looking up the procedure source information, but one needs to know the airport and the procedure to do that.
" To understand if it might be a charting error, I start by looking up the procedure source information, but one needs to know the airport and the procedure to do that.
flyingcheesehead
Touchdown! Greaser!
Huh. Interesting... I just had to check my FOM to make sure it wasn't taken out and I missed it (which is not beyond the realm of possibility).
We still have it in there.
Yep, we still have it as well. Pretty much looks like it's word for word what @John Collins posted, which was probably copied from the OpSpec C073 on FSIMS (or whatever they call the thing that replaced it).
BTW @John Collins you are a wealth of knowledge on this stuff. How did you learn it all so well? Do you read the TERPS weekly or something? Impressive!