View Full Version : DH v MDA

22nd Aug 2005, 18:23
Perhaps some clever bod, can answer this for me, and provide a reference in PAN ops or summit simular.

On a EFIS aircraft, ie G5, there are two options on the Display controller panel, for setting minima on the approach.

Do these numbers have any other function than a display on the PFD with either DH/MDA and the selected value.

One is labeled DH the other MDA.

In the past I would have always said DH is for a precision approach ie PAR/ILS, and MDA for a non precision approach.

However I have been instructed that now the DH is only for Rad Alt specified decision height approaches, ie Cat 2 and above.
MSA should be set for CAT 1 ILS aproaches.

I would appreciate a tie breaker for this question please.

As the EFB on my AC does not have chapter and verse of the regs installed, I am at a loss to answer the question that I was asked, and I dont want to be stupid all my life.... Windy.

Human Factor
22nd Aug 2005, 18:32
No expert, but DH is a "Height", ie. above the ground.

MDA is an "Altitude", ie. above a specific datum (MSL usually).

Therefore my guess would be that unless you operate using QFE, which most operators don't, DH is only valid using the RadAlt.

22nd Aug 2005, 18:38
isnt that where it gets its feed from anyways?

22nd Aug 2005, 20:01
Itswindy - as HF says, there is an element of confusion in your query.

If you have 'DH' for precision apps you should also have 'MDH' for NPAs which means you are using QFE.

If you have 'MDA' you should have 'DA' which means you are using QNH.

I assume 'MSA' is a typo?

To my knowledge I do not know of MDA (or MDH) EVER being set for a CAT1 ILS.

One can, of course, set RadAlt DH when flying to MDA.

Right Way Up
22nd Aug 2005, 20:43
DH setting is used when flying an approach based on Radalt i.e. Cat2/3 ILS.
MDA is used when flying an approach based on Baro alt i.e. Non-precision or Cat 1 ILS. This is based on operators using QNH for approach. I have never used QFE so am not sure if there is a difference.

22nd Aug 2005, 21:33
MDA is used when flying an approach based on Baro alt i.e. Non-precision or Cat 1 ILS - are you QUITE sure of that statement???

22nd Aug 2005, 22:40
A fairly simple and physical thing at work, I suggest ..

If the minimum level is going to occur

(a) over the runway, it makes sense that it be radalt based, particularly as one prefers a higher accuracy as the distance to the hard bits below decreases. However, one needs to be over a known ground datum for the thing to make repeatable sense. In the case of cat 2/3, where the aircraft will be over the runway or immediate runway end environment and the position at minimum is known with a reasonable accuracy, it works fine. Given the aim of a cat 2/3 approach is to push the visual environment to critical limits, using a baro decision would make little sense, given altimeter errors.

(b) somewhat prior to the runway end, local terrain is going to complicate procedures sufficient to the extent that a radalt alert has not much meaning other than for general terrain proximity warning where that might be appropriate. Hence the baro decision for minimum is appropriate. While a cat 1 ILS is a precision approach by definition, generally it terminates somewhat prior to the runway end with the necessary need for baro rather than radalt decision.

Procedurally, for (b), some operators get rid of the DH alert altogether, others use it as a backup to the baro decision provided that the terrain environment makes that philosophy sensibly workable - otherwise it is wound out of the scenario.

I was brought up on this latter philosophy and thought that it worked fine .. it always seemed sensible to me to use whatever warnings and alerts were useful to maximise the likelihood of the crew's maintaining SA during the most critical stages of approach. However, I do acknowledge that my views necessarily are biased due to that routine exposure.

22nd Aug 2005, 22:52
Airbus FBW A/C procedures are to set DH for the CAT2/3 approaches into the DH field.

For CAT1 set DA into the MDA field.

For N/P approaches set MDA into the MDA field.

22nd Aug 2005, 22:59
Decision Height was generally used for the definition of the criteria for aborting, i.e. initiating missed approach, during an approach with vertical guidance, i.e. when on a glide slope. The FAA now wants to call it Decision Altitude. The idea is that you follow the glideslope, and initiate missed approach at the DH if the runway environment is not at that moment visible.

Minimum Descent Altitude (MDA) was/is used when an approach did not include a glide slope, i.e. the basis would be to descend from the fix to the MDA. Then motor towards the airport at that height until the decision point, which in this case is normally defined (on a straight in) as not seeing the runway in time to perform a reasonable landing.

A reference from the FAA Aeronautical Information Manual:
http://www.faa.gov/ATpubs/AIM/Chap5/aim0504.html#5-4-19 :

"4. Chart Terminology

(a) 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.

(b) Minimum Descent Altitude (MDA) has been in use for many years, and will continue to be used for the LNAV only and circling procedures. "

A Cat I ILS then, even if not using a radalt, would have a Decision Height. These days, of course, almost no one operating to Cat I or below does not have a radalt, so it commonly is interpreted as a radalt height.

The EFIS, of course, can display the limit either on the basis of radalt or baro alt, so may incorrectly use the term MDA for the selection of baro for the bug height. This would thus also work for a circling approach, as the EFIS will command level flight at the MDA in that case. If you are on a glideslope, you should be initiating missed approach when reaching bug height anyway.


22nd Aug 2005, 23:19
Oh I am so afraid he actually is. It is my understanding we could stick to MDA / DA / DH designation for the different types of approaches just because that's what they are. However the Bus displays the numbers on the FMA as MDA or DH only, and indeed the methodology is DH for radalt-controlled approaches and MDA for all the rest. :oh: To induce a little comm mayhem on the flightdeck I keep pronouncing the approach C/L item "MDA... ___ set [both]" as "decision altitude" for ILS CAT I just because MDA IS something completely different. :confused:

On the other hand I must say that in fact Airbus advocates that non-P.A. are flown under constant angle with VNAV help and MDA be treated as DA, i.e. no leveling to MAPt but immediate G/A. Although quite different from the Doc8168 concepts I struggled so hard to embrace, for straight-in approaches I believe it is a major step forward, safetywise.

Anyone cares to explain in little detail the Visual Descent Point concept I believe to be in the US?

(the un-real)

Right Way Up
22nd Aug 2005, 23:49
I am afraid that the manufacturers terminology is at odds with ICAO approach terminology. The DH setting gives you a minimums callout based on radalt, and the MDA gives you a callout based on Baro. Unfortunately that terminology then is at odds with a Cat 1 decision which should be called a DA, but uses the MDA setting as it is with reference to baro alt.

23rd Aug 2005, 06:47
It was a long day, a non English keyboard, and strong beer.....

The question was raised to me as a result of the use of DH for QRE as the word HEIGHT.

The FAA reference at least gives a point to hang my hat.

GUCLU: I will continue to use your system untill proved to be wrong, (in G5)

Thanks guys....

23rd Aug 2005, 07:59
Thanks, RWU for the explanation. My head now hurts!:D

What hope do we simpletons have when manufacturers depart from 'standard'? How DID these aircraft achieve 'certification'?

BTW, can someone explain 'G5'? Is it the Gulfstream, and are all 'busses' the same?

23rd Aug 2005, 08:54
What hope do we simpletons have when manufacturers depart from 'standard'?
I might be wrong about this, but I suspect it all started with the FAA's previously preferred use of the expression "DH" when everyone else was using "DA" for Cat 1 ILS. Methinks that manufacturers were simply following the path of least resistance for the sake of achieving FAA certification of their aircraft and/or components. I for one am happy to see that there is now, apparently, a will within the FAA to bring this terminology into line with ICAO. I foresee a scramble by manufacturers, to catch up with this.

23rd Aug 2005, 09:19
My head hurts too.

so many Glfs and so little time....

Still thanks for input guys...


23rd Aug 2005, 12:23
As far as DA/DH is concerned, take whatever the approach charts (Jeppesen) tell you to use. Have you ever noticed how minimums are depicted? Like DA (H) 372' (200') on a CAT I approach. The DA is bold printed and the DH is in parenthesis for a reason. That tells you to take DA. On a CAT II approach you will find an RA. Therefore, use the radar altimeter on that one. Well, thatīs what Iīve been told to do when in doubt.

Or is the confusion basically when to use the radar altimeter instead of the baro altimeter? As before, the approach charts tell you when to use the RA. If it doesnīt say RA, use the baro altimeter.

23rd Aug 2005, 13:26
It's actually quite simple....

If you fly on QNH, like most people these days, and you are doing a non-precision approach or a Cat 1 precision approach, you set the baro minima for the approach as given by the bold print in the Jeppesen or Aerad or whoever) chart. For a non-precision approach this is a Minimum Descent Altitude and for a precision approach this is a Decision Altitude. If you have a radalt (whether part of the EFIS or a separate analogue indicator) on which you can simultaneously display a radalt minimum, then you can of course set the equivalent value from the chart (or better, something like 50 feet below for non-precision or 20 feet below for precision, to account for uneven terrain on the final approach track. Whether you do this will depend on your Company procedures.

If you are flying a radalt-based approach such as a Cat 2/3 ILS or an offshore airborne radar approach to an oil platform (though I guess you wouldn't do this in your Bus or G5), set the radalt to the Decision Height or Minimum Descent Height and ignore the baro box.

Right Way Up
23rd Aug 2005, 13:35
If you have a radalt (whether part of the EFIS or a separate analogue indicator) on which you can simultaneously display a radalt minimum, then you can of course set the equivalent value from the chart (or better, something like 50 feet below for non-precision or 20 feet below for precision, to account for uneven terrain on the final approach track.
That technique is an accident waiting to happen, and as my previous company found out the CAA are not too impressed if operators use radalt callouts for baro based minima.