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IAPs with inexplicably high minima - why - and why is ILS 250ft DH instead of 200ft

Could it be that the DA is at 269 feet as there is an obstacle (lighthouse) at 210ft? See AIP chart.

Wolfgang

EGTF, EGLK, United Kingdom

It sounds like this is more complicated…

Administrator
Shoreham EGKA, United Kingdom

It is also related with obstacle clearance during go around.

LPSR, Portugal

I wonder if @ncyankee might have an idea, even though this is PAN-OPS and not TERPS.

Administrator
Shoreham EGKA, United Kingdom

I have not studied PAN-OPS, but two main reasons for increasing the DA relate to:

1) Close in obstacles that penetrate the GQS
2) Obstacles on the Missed approach course

When a flying a vertically guided procedure, the decision is made at the DA/DH, but physics dictate the aircraft momentum will carry the aircraft below the DA/DH before the descent can be arrested and climb has begun. This has to be accounted for on the go around. So one does not want to sink into obstacles or not be able to climb at the minimum climb gradient and have issues with getting too close to the obstacles on the miss. In TERPS, they will adjust the DA/DH to account for this, either by a fixed adjustment or projecting backwards along the GS to a point where the DA/DH will put the obstacle in the visual segment where it can be visually avoided.

KUZA, United States

An extra 50ft does however seem quite a bit, looking at what is in the vicinity at LFAT.

Administrator
Shoreham EGKA, United Kingdom

LPSR, Portugal

I often wondered why the UK has DHs like (EGMD ILS21)

in the absence of any evident obstacles, when the rest of the universe has simply 200ft AAL.

Apparently it is the assumed missed approach performance which drives this stuff. This is EGKA 02, where the hill is very far ahead…

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I often wondered why the UK has DHs like (EGMD ILS21)

ILS21 OCA = 430ft is probably higher as it’s offset ILS not stright-in (due to Danger Area D141 which I was told sits at close as 3/4 LLZ scale deflection, someone tested that for sure), 100ft agl obstacles punching from bellow at 1DME on tight missed approach turn away from DA044 and 400ft agl windfarm, funnily enough NDB21 OCA = 420ft, go figure

I did one IMCr revalidation there, went missed somewhere near 750ft with all safety addons just after we reported 3nm

Last Edited by Ibra at 20 Jul 15:05
Paris/Essex, France/UK, United Kingdom

Didn‘t do the math, but missed approach track says „turn right as soon as possible“ which takes you directly above a 110ft obstacle. So OCA of 430 feet doesn‘t sound too wrong. 200ft would be clearly too low (and is typically only found at approaches where the missed says „go straight ahead (over the runway) when the tower is far enough from that runway)

Germany
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