Help settle an argument about DA/MDA
Joined: Apr 2007
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From: Center of the Universe
DAs/DHs aren't normally increased due to obstacles that occur on final approach prior to system minima as any such obstacle would probably demand an increased GP angle. Increased DAs/DHs are far more common due to problems in the missed approach segment which for a precision approach commences at system minima, i.e. the 200ft/0.65nm point.
Joined: Apr 2009
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From: Hotel Gypsy
EN48, think of it this way.
Let's assume the ideal GP is in the 3.0deg territory. Whichever way you look at things, you will be at 200ft at 0.65nm, regardless of obstacles. Increasing DA will only shift the MAP further away from the runway (you'll still be following a 3.0deg GP) and consequently, the only reason you would want to do this would be due to an obstacle beyond/below the 200ft point. For any IAP design, the MAP segment takes primacy as it requires a climb gradient of 2.5% which is a more shallow angle than the GP (about 5%) and consequently is more susceptible to obstacles. (OK, there's a level bit of the MAP segment after MAP and before commencing climb but, for ease, the numbers still favour the more shallow gradient)
PS. Good comment on pilots understanding TERPS (other versions are available
).
Let's assume the ideal GP is in the 3.0deg territory. Whichever way you look at things, you will be at 200ft at 0.65nm, regardless of obstacles. Increasing DA will only shift the MAP further away from the runway (you'll still be following a 3.0deg GP) and consequently, the only reason you would want to do this would be due to an obstacle beyond/below the 200ft point. For any IAP design, the MAP segment takes primacy as it requires a climb gradient of 2.5% which is a more shallow angle than the GP (about 5%) and consequently is more susceptible to obstacles. (OK, there's a level bit of the MAP segment after MAP and before commencing climb but, for ease, the numbers still favour the more shallow gradient)
PS. Good comment on pilots understanding TERPS (other versions are available
).
Thread Starter
Joined: Aug 2001
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From: Cornwall
And finally.....
My colleague has read all contributions to the thread and we have made peace over the subject of rad alts and Class 1 ILS approaches. I have to make a small correction to my original post and point out that he did not say you must use the radar altimeter readings for a DH reference.
In view of the huge amount learnt from all your submissions a truce is called and the only whisky involved will be a shared glass of single malt - after work of course.
G.
IMHO this is when Prune works best - not a single swear word or raised voice, a little bit of sarcasm here and there but otherwise an excellent discussion with a little hint of humour and a great deal of enlightenment
In view of the huge amount learnt from all your submissions a truce is called and the only whisky involved will be a shared glass of single malt - after work of course.
G.
IMHO this is when Prune works best - not a single swear word or raised voice, a little bit of sarcasm here and there but otherwise an excellent discussion with a little hint of humour and a great deal of enlightenment
Last edited by Geoffersincornwall; 11th August 2015 at 06:17. Reason: final point
Joined: Dec 2006
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From: UK and MALTA
HI CGB and others. Firstly thank you for the comprehensive explanations you have provided and it all makes sense to me.
In Europe the Jeppesen plates generally include a TCH ( threshold Crossing Height) based on the theoretical GP angle and the point the G/S impacts the runway. As the optimal TDZ is normally 300m from the threshold the 1:60 rule, for 3 degrees, puts the G/s 50 feet above the threshold. Consequently most 3 degree slopes, without any GA infringements quote a TCH between 45 and 55 feet.
As the aircraft descends along the G/S the crew make the decision as the BARALT approaches/reaches the DA minima. On the 3 degree path this theoretical point is 1200m approximately from the optimum TDZ point.
Therefore, unless the ground is significantly higher than the threshold, out to about 1km from the threshold, which would be highly irregular, the practice of setting a backstop DH 30 feet below the approach DH minima, works very well.
I have never operated in the US or much in very mountainous regions so this may not work everywhere. However, my natural sense of safety/justice would be seriously challenged if I thought at a theoretically correct DA (no flight path error) I was significantly lower to the surface than the procedures published DH.
In any event, getting lower than 170 feet from the surface IMC during the approach is probably close enough for me and I would really want to be going around at that point (accepting I will continue to descend below 170 feet momentarily in the go-around action). I may be a little naive in this respect but at a 500 fpm ROD I am a little less than 15 seconds from impacting the surface.
Interesting thread though that does challenge the concepts we all carry in our heads.
In Europe the Jeppesen plates generally include a TCH ( threshold Crossing Height) based on the theoretical GP angle and the point the G/S impacts the runway. As the optimal TDZ is normally 300m from the threshold the 1:60 rule, for 3 degrees, puts the G/s 50 feet above the threshold. Consequently most 3 degree slopes, without any GA infringements quote a TCH between 45 and 55 feet.
As the aircraft descends along the G/S the crew make the decision as the BARALT approaches/reaches the DA minima. On the 3 degree path this theoretical point is 1200m approximately from the optimum TDZ point.
Therefore, unless the ground is significantly higher than the threshold, out to about 1km from the threshold, which would be highly irregular, the practice of setting a backstop DH 30 feet below the approach DH minima, works very well.
I have never operated in the US or much in very mountainous regions so this may not work everywhere. However, my natural sense of safety/justice would be seriously challenged if I thought at a theoretically correct DA (no flight path error) I was significantly lower to the surface than the procedures published DH.
In any event, getting lower than 170 feet from the surface IMC during the approach is probably close enough for me and I would really want to be going around at that point (accepting I will continue to descend below 170 feet momentarily in the go-around action). I may be a little naive in this respect but at a 500 fpm ROD I am a little less than 15 seconds from impacting the surface.
Interesting thread though that does challenge the concepts we all carry in our heads.
Joined: Apr 2007
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From: Center of the Universe
Increasing DA will only shift the MAP further away from the runway (you'll still be following a 3.0deg GP) and consequently, the only reason you would want to do this would be due to an obstacle beyond/below the 200ft point.
Joined: Oct 1999
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From: Den Haag
My colleague has read all contributions to the thread and we have made peace over the subject of rad alts and Class 1 ILS approaches. I have to make a small correction to my original post and point out that he did not say you must use the radar altimeter readings for a DH reference
Thread Starter
Joined: Aug 2001
Posts: 1,308
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From: Cornwall
212
Yes..... it makes me want to spit feathers ?@*&!***
Like having three automation related buttons labelled 'NAV' and critical power supply switches located next to each other and given potentially ambiguous names. (Main???? Master ?????) - non English speakers beware and to top it all an 'LNAV' button that delivers 'FMS' and a 'NAV' button that delivers 'VOR'.
I think the design team must have been out to lunch that day
G.
Like having three automation related buttons labelled 'NAV' and critical power supply switches located next to each other and given potentially ambiguous names. (Main???? Master ?????) - non English speakers beware and to top it all an 'LNAV' button that delivers 'FMS' and a 'NAV' button that delivers 'VOR'.
I think the design team must have been out to lunch that day
G.
