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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Forgive me chaps if my knowledge base is out of date (being three years retired now) but my understanding was that you must NEVER dip below the published MDA published for an NDB approach; and despite a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?"

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