Alpha_Floor wrote:
This plate for GCTS NDB Rwy 07 makes a distinction between CDFA and non-CDFA (i.e., dive and drive)
Interesting… NDB approach that uses GPS points and distances.
Peter_G wrote:
he emphasises that under ‘Dive & Drive’ you can settle down to MDA earlier with the ability and more time/chance to see the PAPI’s further out (than under CDFA): A major factor and important aid in low visibility
D&D has the visbility benefit no doubts and it works perfectly over the sea with no obstacles & visible land marks, but short 1km runway it maybe wise to have 1st D&D approach to see/orientate & judge wind/gusts, then have 2nd D&D or CDFA approach to land on it, it maybe hard to do both on the 1st D&D shot in hastly manner !
D&D still require lot of skills if the runway is spotted a bit late: landing in 1km runway from 300ft agl over it’s treshold in nil winds is doable in any SEP but need lot of regular practices in VMC
D&D with 300ft MDH on 750m runway length needs unorthodox ways of creating drag, they will rarely teach these in IFR courses as flying is done to 3km runways Rate1 turns, 1.3*VS approach and slipball in the middle
D&D with 300ft MDH on less than 500m runway is a suicide, except for IFR in C182 
Do you not normally add 20ft to a MDA but not necessary to add the 20ft on a CDFA?
Personaly, I prefer CDFA to D&D but I know there are many others that think the opposite. For me its whichever you feel more comfortable with if you are given a choice.
Don’t forget that PAPI’s are calibrated for an aircraft of a particular height, eg a 737 or 757 or whatever is the most important CAT to the airport.
gallois wrote:
Do you not normally add 20ft to a MDA but not necessary to add the 20ft on a CDFA?
I had the impression (UK only), one will have to add 50ft to get DH for an NPA flown as CDFA to avoid going bellow MDH due to decision lag and inertia, on PA you should not add anything but again UK traditions: adds 50ft ALT PEC (if not in POH), 50ft inertia, 200ft for IMCr, 50ft for IR rated…
One can just fly the plates numbers rounded to system minima and fly them like Americans fly their Jepp plates down to numbers or max CDI deflection which ever comes first, rather than getting busy adding 20ft & 50ft here and there depending on this and that: first, it makes no practical difference and second, just adds to confusion and workload, I think I can come up with 240ft = 270ft+50ft+20ft with an examiner sitting nearby
How does it work in France for add-ons on ILS DH? LNAV/VOR DH on CDFA? or MDH on D&D?
Ibra wrote:
I had the impression (UK only), one will have to add 50ft to get DH for an NPA flown as CDFA to avoid going bellow MDH due to decision lag and inertia, on PA you should not add anything but again UK traditions: adds 50ft ALT PEC (if not in POH), 50ft inertia, 200ft for IMCr, 50ft for IR rated…
I believe 50 ft are added onto an MDA (minimum descent altitude) to ensure that one will not dip below it during the go around. However, a DA (decision altitude) already takes this into account hence the addition is not required, it’s the altitude where the decision is made, not the altitude “by which the decision must have been made and the aircraft be already climbing”. I was taught to add 50 ft anyway (don’t know why, maybe because traditions?). So I just default to always adding 50 ft on MDAs or DAs, whether NPA or PA.
When an NPA is flown using the CDFA technique, and an MDA is published, I would add 50 ft to the MDA and that would become my DDA, Derived Decision Altitude.
I believe there is a point to make here: what if by adding margins to the DA where they are not required, this ends up causing you a go around from a very low height which is arguably less safe than simply descending to the published DA and potentially making visual contact then?
That 200ft figure for IMCr seems rather excessive in my opinion. Is this a mandated requirement or just something people do/teach in the UK?
For a short runway without approach lights, DND provides one much more time to get stabilized at the MDA and acquire the runway when the visibility is close to the minimums. With CDFA, it may boil down to an instant in time and at a higher derived DA, typically MDA+50. No one should expect to land if they first spot the runway at or near the threshold, that is not the purpose of the MAP. If turns are involved in the missed approach procedure, they are not to commence until after passing the MAP.
Take this approach at Twin Lakes NC, 8A7, the runway is just under 900 meters. One would need the visibility to be at least 1 3/4 SM to expect to see the airport by the time of reaching the DDA, so if the actual conditions were 1000 overcast and 1.5 mile, one would have to miss the approach at the DDA. The circling altitude is essentially the same as the DDA. So if you descended to the circling altitude and spotted the runway 1.25 miles out, you might be able to make a steep descent to runway 9. Power off in my Bonanza is about 8 degrees, so even with reduced power, a 6 degree descent is possible. The PAPi is 3.5 degrees to runway 9 and there are obstacles if you follow the advisory glide path below the MDA, so definitely a 3 degree slope is not a good idea. The alternative would be to continue at or above the circling altitude and enter the pattern from overhead the airport to a left traffic and fly a close in pattern to keep the airport environment in sight. With the stated conditions, one could fly the pattern as it is in class G airspace and VFR conditions below 700 AGL.
I used ForeFlight to obtain the 3D view of the airport at 1.5 NM (1.73 SM) on a 3 degree slope. I also attached the approach chart,
Alpha_Floor wrote:
So I just default to always adding 50 ft on MDAs or DAs, whether NPA or PA.
I think that answer would make everybody happy (altough reasons behind are different in the 3 possible cases )
Alpha_Floor wrote:
That 200ft figure for IMCr seems rather excessive in my opinion. Is this a mandated requirement or just something people do/teach in the UK?
It’s recommended aerodrome minimum which give ILS at 500agl and NDB at 600ft agl, an IMCr holder can legally fly system minima on his licence, however, I recall IMCr skill/revalidation test is made with extra add-ons and examiners gets senstive about it unless it’s joint IR/IMCr signoff, none of this make any difference if you go for revalidations with ILS/NDB at Lydd 
In practice, currency is king (good autopilot ), if an IMCr holder flies CDI in the middle down to ILS200 why he should go-around at 500ft? with all risks of go-around and diverting? what about non-current IR holders who can’t keep his CDI in the middle down to ILS200
NCYankee wrote:
No one should expect to land if they first spot the runway at or near the threshold, that is not the purpose of the MAP
Indeed, it’s not for landing
NCYankee wrote:
The circling altitude is essentially the same as the DDA
On this, had the impression UK law permits going down down to DH/MDH/DDH first before recovering to the to higher circle-to-land minima at the MAPt (examiners expect this and they will say so in the breif), but yes under FAA rules one is not permitted to go bellow circling minima if the plan is to circle to land…
Emir wrote:
Interesting… NDB approach that uses GPS points and distances.
I’m pretty sure those waypoints are not part of the actual approach. Jeppesen uses them for the “overlay” approach in their nav databases and they are charted for the benefit of pilots who are using those databases.
Alpha_Floor wrote:
I believe 50 ft are added onto an MDA (minimum descent altitude) to ensure that one will not dip below it during the go around. However, a DA (decision altitude) already takes this into account hence the addition is not required, it’s the altitude where the decision is made, not the altitude “by which the decision must have been made and the aircraft be already climbing”. I was taught to add 50 ft anyway (don’t know why, maybe because traditions?). So I just default to always adding 50 ft on MDAs or DAs, whether NPA or PA.
Part-NCO is not clear about the need – or not – for adding something to the MDA to obtain a DDA (Derived Decision Altitude). What is clear is that there is no danger in dipping below the MDA during a go-around and indeed there is a rule change in the works that would explicitly say that no add-on is needed.
More info in my article here.
Here, if you are descending to a DA/DH no addition is required. ie ILS or CDFA.
If descending to a MDA/MDH you have an add on so as not to descend below the MDA.
I thought that add on was 20ft here but I will have to double check that as the posts here have put a doubt in my mind:)
