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-   -   NDB approach - Do you ADD an allowance for "sink" ? (https://www.pprune.org/tech-log/10937-ndb-approach-do-you-add-allowance-sink.html)

Tartan Giant 2nd May 2001 00:46

NDB approach - Do you ADD an allowance for "sink" ?
Given an ILS DA has an allowance "built-in" for a shallow dip below the published DA during a GA, I am surprised to learn that a UK company has added 50' to the MDA for NDB approaches, so there is less chance their aircraft dip below the MDA during a GA from "limits".

Forgive me chaps if my knowledge base is out of date (being three years retired now) but my understanding was that you must NEVER dip below the published MDA published for an NDB approach; and despite a definitive figure published in the appropriate manual, there was nothing about adding this 50' business to any MDA so published.
You were left to your own skills to NEVER go below the MDA for an NDB.

Is there any other company out there adding this "sink allowance" of 50 feet to the NDB minima ?

Best wishes,


fireflybob 2nd May 2001 00:51

I think it depends on how you fly the approach. If you are intending to fly level at MDH/A (until the MAP) then you do not need to add an allowance.
However, if you are flying a "stabilised" approach and are intending to immediately Go Around on "approaching" MDH/A then, yes, you should add the aircraft allowance.
In either case you should, obviously, not got below MDH/A without the required visual reference.
Hope this helps.


Propellerhead 2nd May 2001 01:17

I agree with FFBob above. We add 50' to the MDA in our airline. In a jet you don't fly level at MDA, you fly the approach as you would an ILS, and initiate the GA at 50' above MDA (from a 3 degree descent), therefore ensuring you don't go below MDA. The logic is that if you were to fly level, when you broke out of cloud you would be too high too make the approach (due to the high speed), so we fly a stabalised descent as close to a 3 degree slope as we can. Most airfields we go to have an ILS, so NDB's are usually only used if the ILS is on maintenance on a nice day, so it becomes largely a visual affair anyway. Have yet to do an NDB down to MDA.

Mr moto 2nd May 2001 01:43

A company, by way of it's SOP's, has the right to impose its own minima for whatever type of approach.

However, our SOP makes no reference to this and as the MDA/DA varies with category I would be inclined to leave it at that. Initiate go-around at DA on a precision approach and not go below MDA on a Non-precision.

HighSpeed 2nd May 2001 09:45

I personally tend to disagree with flying an NDB approach like an ILS approach. I believe the NDB approach is design to safely bring one below cloud cover in order for one to sight the runway. If one flies a 3 degrees descent, the chances of obtaining visual will be less plus the fact there is absolutely no guarantee the 3 degrees takes one to the threshold!!

In a 'semi-precision' approach such as VOR/DME, then it would be a different story...


yowie 2nd May 2001 10:10

We used to add 50ft for pressure error correction(PEC)in piston ops(usually as defined in flight manual)o but turbine A/C appear to not require it,probably due to more sophistication of systems.As for flying a 3deg slope,OK if NDB location allows to position the A/C on a final approach,totally useless when aid is at other end of the runway.A fly level approach(IMO)means plan to reach MDA/minima as soon as established inbound and continue at that level to MAPT to best utilise the available circling area.

the force,it really sucks

Iz 2nd May 2001 15:38

Highspeed, virtually all NDB approaches in (Western?) Europe are designed with a 3 degree descent path in mind.
If the weather is just above NDB minima, with the NDB MDA being less than 500' AGL in many cases, would you fly straight and level in search of the airport?
What would the purpose of the "being established at 500 feet" SOP be? You're leveling off at say 475' AGL, adding thrust, then when you spot the runway after a mile of level flight, you cut the power again and pitch the nose down?
If the weather's so bad that it's even doubtful you'll see the runway at MDA, and an NDB approach is the only thing available, it might not be a good idea to go there in the first place.


Tartan Giant 2nd May 2001 18:02

Thanks chaps for the answers so far.

I can appreciate the arguments creaping in here about the "fly level" and "make it like an ILS" sort of problems, but I was just after a few names of WHAT CARRIERS IN THE UK add 50' to the MDA, when forced to do an NDB.

On passing, during my career (38 years) I had both the "fly level" operation and then the new thinking "pretend it's an ILS" so I know where the thoughts are coming from.

I know you must follow your company SOP's and of course any Commander can add a sensible ammount to both the Altitude and indeed the RVR to make his own limits on the day, no matter what the "plate" says. I have done it during TS's in Eastern Block countries (where the quality of the NDB flights checks are perhaps questionable)and where the arbitory minima presented makes little sense on the day.

I just want to know why there is a question mark over the 50' add-on between companies, and what companies say YOU MUST add this 50 feet (using "pretend" GS angles > 3.4 degrees).

Looks like a gap in CAA approvals for the AOC in terms of the NDB approach ?

For instance FIREFLYBOB says, "it depends how you fly the approach" - with respect, I have not seen that in any CAA / Company Ops Manual. One day you might like to "fly-level" at MDA, the next "I'll pretend it's an ILS" and add 50'.
Don't get me wrong FFBob, your information did help, but you see where the problem arises.

Best wishes,


Bored Cheese 2nd May 2001 19:47

The practice of flying a continuous descent during a non-precision approach is in order to prevent approach and landing accidents resulting from destabilised approaches.
Fine in a piston or light turbo-prop twin to level at MDA plus a smidge but jet-ops normally demand that the aeroplane is fully configured, speed and flight path stable with landing checks complete by 500' A.R.T.E.

Having waffled on about the above how do we explain circling approaches and what about contingencies for engine failure during level segments with full flap, gear down etc

E120 2nd May 2001 21:26

Hey Propellerhead & Iz; does "XL" rings a bell? Now for the rest of us; in my company's SOPs is nothing to be found about adding 50 ft. to the MDA. However we fly a non precision approach stabalized on a 3 degree glide path (at least some try to). Going below your MDA during a go-around is a bust and a failure, so what most pilots do (and it,s not written down), is to add approx. 50 ft. to the MDA and innitiate the go-around at that altitude to prevent busting the MDA. The "50 ft.- rule" works for most turboprops and jets. Hope this helped.

Greez, E.

411A 2nd May 2001 21:29

Well now, landing flap is normally not selected during a circling approach until landing is assured. This creates a larger circling radius but DOES cater for the engine out performance data. Many airlines have discontinued circling approaches in heavy jets because of the problems involved.

Flanker 2nd May 2001 21:42

JAR requirements now include a circling approach here in Europe as part of our Operators Proficiency Check.
It is a slightly contentious issue amongst some trainers.

PPRuNe Towers 2nd May 2001 23:22

Some of our Canadian readers might be able to comment on this subject.

As I understand it, companies can be signed off on an individual basis to operate pseudo 3 degree non precision approaches to MDA without any buffer added. The 'dip down' a la ILS is is considered adequately covered by the company's SOP's, audit performance and record of safety.

Regards from the Towers
[email protected]

Iz 2nd May 2001 23:27

Hmm Propellerhead, any idea what E120 means? Are you talking about Exel? If not, please elaborate.

411a, Flanker's right, had to do circling on my JAA type exam on the 75.

411A 3rd May 2001 05:44

The FAA also REQUIRES a circling approach for an IPC or type rating. It is "normally" done with all engines operating but many check airmen (and the FAA) want to see one with an engine inop. However, some airlines prohibit circling approaches in heavy jets during normal line operations. Considering the problems involved with a B747 circling at 400 feet, maybe not a bad idea, especially with the younger guys with EFIS (only) experience. The older guys have done this for years. :)

Iz 3rd May 2001 11:42

On the subject of not being established at 500' on the approach:
Difference between levelling off at MDA and circling is that circling is a purely visual maneuver. It's just like part of a traffic pattern. Since circlings are very rare anyway, I wonder how many of you would do a circling 50 feet below cloudbase. Wouldn't think so.
And it's common sense to make a decision on maneuvering your 747 at 400 feet above the ground.
By the way, in our company, circling is done mostly on autopilot.

scrubba 3rd May 2001 19:49


what has EFIS got to do with it? i regularly fly circles in both EFIS and round dial but i haven't noticed any EFIS-induced problems. am i missing something?

"As you now appreciate, termination is in your own best interest..."


fireflybob 3rd May 2001 22:52

Tartan Giant - yes, when I said "it depends which time of approach you fly" I was not suggesting that pilots for an operator had much of a choice on the matter, although, of course, circling approaches are somewhat different.

Perhaps it would have been clearer if I had said "it depends on which type of approach the operator wants it's pilots to fly".


Tartan Giant 4th May 2001 03:35


Thanks for that........yes, as you say, it's what the particular operator wants.

Best regards,


411A 4th May 2001 07:31

You are indeed fortunate that you have conventional "round dial" experience and therefore would expect that no problems would follow. Having trained young guys that have been in glass for a few years and then revert to round dials again, due to upgrade, the change is a very big problem. Their scan goes out the window because they tend to fixate on one instrument, to the exclusion of others, thus extending the program to a large degree. The old saying that a picture is worth a thousand words sure is correct when these guys look at the instruments and proclaim....."where the hell am I?"

OzExpat 5th May 2001 19:31

Sorry Tartan, I can't help with your specific question, but can give everyone else a bit of perspective on the situation in this pocket of the planet. First off, no operator I know of requires an additional 50FT for "sink thru" on any NPA around here.

Unless your aircraft's PEC is "zero" you need to adjust your MDA accordingly. In our official publications we're told that, if we don't know the extent of PEC, we need to add an arbitrary 50FT to the MDA.

Next point is that many pilots, myself included, prefer the 3-degree approach due to the simplicity of it and the options and flexibility it provides. It facilitates a stabilised approach, is more fuel efficient, kinder on the engines and puts you in a good position to go around and/or handle an engine failure.

It isn't especially difficult to figure how far from the THR that the MDA intersects the real 3-degree approach path to the THR. Having established that position, check for any limiting descent steps on final approach and make any allowance necessary. Then just work backwards to find out what altitude you need when joining the final segment. This is sometimes awkward, depending on proximity to terrain, but can usually be compensated quite readily with temporary use of a 2x profile.

While this can all be worked out quite readily when there's a DME available, or if you have the properly certified GPS gear, or INS etc., it ain't much more complex when you have to DR the distance aspect.

This is because the chart shows you the outbound timing, which you can adjust to take account of headwind or tailwind. Whatever effect the wind has on the outbound leg, it'll have the opposite effect on the inbound - unless it's a straight crosswind, of course. Anyway, the point is that you'll have a pretty good idea how much time will be available for the inbound leg. This, in turn, helps you to identify the time available to descend, so as to reach MDA near enough to the point of interception of the 3-degree approach path to the THR.

Once you have that point worked out, in terms of actual distance, or just time, you can fly the NPA at your 3-degree profile to MDA. If you get visual, you'll be near as dammit to being on the VASI or PAPI GP and that's how it's mostly worked out for me.

The larger operators hereabouts have already worked all of this out for their crews and the info is in their SOPs.

Now, if you don't get visual at MDA ... well, in the B200 I fly, it never gets the chance to sink thru. It's a very responsive aeroplane.

The other good thing about the stabilised approach is that it's better for the comfort of your pax. THEY know when you're on descent so they start to get nervous if the plane levels off and engine noise increases, sometimes massively, just to keep the plane in level flight. Then they all but lose their meal as the plane noses over as soon as the pilot spots a hole!

The pax are the thing that keep us in our jobs, by using our company's services. If they have a good flight, you'll see them again. If they have a bad one, the company loses them - and all their friends and rellies ... except, perhaps, the mother-in-law! :)

There's only other reason I can think of for adding some kind of fudge factor to the MDA. It's sometimes applied as something akin to an F/O MDA. This sort of thing is used in some parts of the world, I understand, http://www.pprune.org/ubb/NonCGI/frown.gif though I have no idea what sort of MDA buffering is used, or even how it's calculated. Maybe someone out there can enlighten me on this?

Dispela olgeta samting i pekpek bilong bulmakau!

Propellerhead 5th May 2001 19:45

Am I missing something. No, I don't know what XL / Exel is?? Am I upsetting corporate jet pilots who regularly fly level? I was talking about an airliner.

[This message has been edited by Propellerhead (edited 05 May 2001).]

mitch3p 5th May 2001 19:58

Our Ops Specs in the USA for my compnay:

NDBs with a FAF add 50ft to the HAT
NDBs w/o a FAF add 100ft to the HAT
VOR w/o a FAF add 50ft to the HAT
NDB and LOC, the lowest vis 3/4 or RVR4000

In addition we have provisions to plan a VDP on every non precision.


BmPilot21 5th May 2001 20:12

At BMI British Midland we add 50' to the MDA. We also fly circling approaches (eg Nice). Had to do NDB's at Stuttgart (although the beacon is so bad we usually end up doing it visually). On the Aerad (Thales) plates we have a vertical profile corresponding to a 3 degree glideslope - PNF reads of the next check height for every mile down to MDA. BMI reccomend full use of the automatics, and it is tought that you use the autopilot (in HDG Select and VS) down to when you are visual. This is due to the high workload involved.

Dan Winterland 7th May 2001 01:49

In my company, the policy is that a non precision approach can be flown using a profile which puts the aircraft lower that the notional 3 degree glide path should the weather conditions suggest you are going to get the visual references late. This allows a level off at MDA (M meaning Minimum and therefore no go below), and continued level flight to the MAP at the approach minimum. If you aim to level off at MDA from a 3 degree path, by the time you have levelled off, you are going to above above the optimum approach angle and may have to go around. This only works with MAPs which co-incide with the 3 degree path, which most do these days, particularly if coupled with DME.

Descending to MDA from the FAF providing you don't exceed RoD limits (which are generous) is allowed, providing you stay in the approach lane. The only exception to this is the SRA where the MDA is calculated using the notional 3 degrees to lower the minima.

DA is the altitude where the decision is to be made. It allows for look up time, time to decide, time for spool-up and time for the aircraft to overcome the inertia of the descent and actually start climbing.

MDA if used wisely can be used to your advantage. Adding extra for dip is adding an extra limitaion and more constraints IMHO.

411A 7th May 2001 05:35

It would appear that only the UK operators add the ubiquitous 50 feet. Are they unable to fly level then? http://www.pprune.org/ubb/NonCGI/eek.gif

wondering 7th May 2001 17:07


once a while I get to fly NDB approaches. And, each time I never get a steady needle. Thatīs why, we fly NDB approaches with the FMS.

So, why would you use heading select on a NDB approach and chase the needle instead of flying with the FMS? Are there any limitations. As far as I can see, the FMS keeps me better on track than Heading Select.

BmPilot21 7th May 2001 23:08

Wondering - Do you fly the 737?

We are not allowed to make FMS approaches - we MUST be in HDG SEL before we intercept the final approach track, be it for an ILS or NPA. The reason being that our FMS position is a combination of IRS position, and radio position. The IRS position obviously drifts, and if we are not getting good radio updates, the FMS position may not be accurate - we would intercept a false localiser / FAT.

On a normal ILS, I usually keep my EHSI in map mode until established, and often see up to a mile deviation. This is why we, unfortunatly, can't fly FMS approaches. Our company Airbuses can, but for us we have to chase those needles!

wondering 9th May 2001 12:38

No, I am not flying a 737. But, the FMS I use gets its position information from radio aids and if no suitable radio aids are available from GPS. From looking at the nav display and the results e.g. being very close to the track, it does a fine job. Still better than trying to fly the needle which swings back and forth all the time. And, coupled with VNAV it flies just like an ILS.

Also, I would imagine that you have to keep your fingers on HdgSel all the itme, since the wind changes with altitude. Anyhow, the workload is reduced and more time is spend monitoring.

Isnīt there an option for GPS on the 737? Now, I am not suggesting GPS as a sole means of navigation, but in most cases it is a lot better than beacons.

BmPilot21 9th May 2001 22:46

As you say, in order to do a FMS approach, you need GPS as a back up for when radio nav. is not available. GPS is available almost all the time - Do you get a warning if GPS is not providing accurate data - must you then discontinue the approach if you are down to 'IRS Nav Only'?

You are right, it does (usually) require constant changing of heading (as it would were you flying manually). However, we do still have a track line which makes it a hell of a lot easier - stick the track line on the EHSI on the head/tail of the beacon, over the inbound course line, and it should fly a perfect NDB!

I don't know if the GPS option exists - probably on the 737NG. Our company Airbus have it.

wondering 11th May 2001 12:47

There is no IRS in my plane. We have something less sophisticated called AHRS. It gives, as far as I know, only attitude information to the PFD but is not linked to the nav system.

In case no GPS data is available a warning/message will pop up in the PFD and the CDU.


We add the 50 for all NPA's so that MDA is treated as a DA.The approach is flown as a stabilised nominal 3 degree slope.We do not add 50 for circling approaches as a level segment to a mandatory missed approach point may be needed to visualy find the field.
To the level sector advocates I would recomend the following.Go and stand in a field two miles from the threshhold ,maybe in a valley with trees,add a gusty wind and low cloudbase.Now imagine a 747 with 300 people on board dragging overhead at 500 or so feet ,gear down,engines howling,IMC,following a wobbly little NDB needle the size of matchstick which may or may not be pointing roughly towards the field if there is no Cb activity!!.Now ask yourself if that really is a sensible method of operating a large commercial jet.

411A 12th May 2001 06:39

Precisely why many aircarriers have eliminated ADF approaches in normal line operations with widebody aircraft. Still a good idea to continue to check in the sim, however.

stormcloud 18th May 2001 02:56

Give me the airbus any time!
Use the autopilot, track and FPA (flight path angle).
Jepp plates have the additions done already so the MDA is the MDA.
Get on the track and select TRK/FPA. The aircraft then flys the track and sorts out its own heading. Half a mile before the TOD, select the FPA as shown on the plate, down you go and just adjust the FPA to fine tune the descent.
Who wants to work hard?

NorthernSky 18th May 2001 04:33

The recommendation to add an increment comes from, amongst others, the IFALPA Working Group on CFIT, and if you don't add the increment, you're breaking the law when you sink below MDA in the go-around.

To my knowledge, Jeppesen plates do not have this increment added, contrary to information above. My company uses Jepps, and we add an increment to all non-precision minima except circling minima, provided the intention is to level at or above the circling minima to maneouvre.

'Brighten my Northern Sky' Nick Drake R.I.P.

EPRman 19th May 2001 15:43

Received an aircrew notice yesterday that with immediate effect 50' will be added to all MDA's. The revised altitude will be called the "Non Precision Approach Decision Altitude". This has come about because,according to the notice,the CAA have pointed out that the figures published by AERAD for non-precision and circling approaches are minimum descent altitudes and not decision altitudes, and our present procedures do little to prevent inadvertent descent below MDA in the event of a go around from a non-precision or the level segment to the MAP on a circling approach.Our SOP's were to fly non-precision approaches as a continuous descent to MDA and to go around at MDA if nothing seen(incidentally until a couple of years ago we used to fly level at the MDA until visual or the MAP).By the way this is at Air 2000.

Tartan Giant 19th May 2001 18:59

Thank you gentlemen for the replies.

The muddy waters are clearing !

Best wishes.


Slick 20th May 2001 00:30

Very interesting thread, but I can't help thinking that adding increments to MDA has been borne about by the ability to do it through automation. Non-Precision 3 degree approaches are not realy that new, I have flown many in older electro-mechanical instrument transport A/C. We never added 50ft to our MDA by moving the little plastic bugs up a bit. I now fly a very mordern A/C (have done for a bit now) and have noticed the only thing that has realy changed is an unhealthy faith in the autopilot system.

If the CAA are so concerned that the British pilot community are busting MDA during the go-around, presumably they will go into print over it, or is this an indivual Ops inspector thing ? MDA is MDA, MIMIMUM.

Ok, I can see the logic it even comes down on the side of caution and thats never a bad thing. However I think flying down to minimums (NPA), on a very dark and dirty night in an airliner, even if its 50ft above MDA is a fairly unwise thing to do, its somthing I would not do unless my life depended on it anyway.

Best regds

stormcloud 25th May 2001 01:46

Obviously a variable as far as the Jepps go. We have plates tailored for the company types and the allowance is added in. Sorry for the confusion.

john_tullamarine 25th May 2001 07:50

An interesting, if somewhat predictable thread, with for and against thoughts as to dive and drive versus stable approach path flight conditions. I declare a bias in that I am fervently for the latter, whenever that option is practicable.

I have always viewed the target height increment as a sensible consideration which, I had presumed, most pilots added informally if not required to do so by formal directive - emergency situations notwithstanding.

I incline very much to OzExpat's philosophy and have only one thought to add to the discussion . ..

From reading in other fields, and my (sometimes bemused) observations from the back seat in the sim, it is very clear that the human beast works best in a "steady" environment, (cf detection of terrestrial threats with peripheral vision in the hunter-gatherer type of chap).

Pilots consistently perform comparatively poorly in error detection and correction tasks if the environment is rapidly varying (typical in a difficult dive and drive situation), yet perform comparatively well if the environment is stable (as in a well disciplined ILS).

I do not suggest for a moment that the former does not have a place in the pilot's arsenal, in the same way that the circling minimum weather approach is, on occasion, very useful. However, if we are inclined to the view that airmanship, basically, is peer-reviewed risk management practised in aeroplanes (and rotorcraft, of course), then we ought to consider preferring the risk averse practice of adopting stablised approaches where such an option is reasonably available.

[This message has been edited by john_tullamarine (edited 25 May 2001).]

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