Maybe I am not correctly understanding this statement, but ... the TERPS criteria allow relatively little change in GP angle in most cases. More likely that the DA would be increased as needed to assure required obstacle clearance and stay close to a 3 deg GP angle. Agree on the MA segment. One reason for increased DA is to meet the standard minimum climb gradient on the MA segment. Some airports have multiple precision approaches to the same runway with different DA's depending on the climb gradient the acft is capable of.

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 ;) ).

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. :ok:

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

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.

Got it! :ok:

Of course, it doesn't help much that more than one manufacturer actually labels the rad-alt bug as 'DH' ...........:E

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.:ugh:

And I thought ergonomics was supposed to be science rather than something made up on the back of a fag packet!

Engineers should not be allowed to design cockpits!