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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)

MAPt 27th May 2001 19:40

My company adds 50' to all MDA`s, to account for sink when initiating a go-around. SOP`s also require a go-around as soon as MDA + 50' is reached, ie. no flying along at that height until the Missed Approach Point. This heightens the need for an accurate glideslope path to be flown so as to avoid unnessecary go-arounds in poor visibility.

BEagle 28th May 2001 03:04

Where an approach requires that, on an NDB approach the aeroplane must not be flown below the equivalent ILS glidepath, then I see some merit in adding something to the procedure minimum as the approach is effectively being flown using the same technique as for a Precision Approach.
However, where this 'notional glidepath' is not mandatory, then you should be free to descend at not greater than the maximum descent profile permitted by the procedure to achieve the 'real' procedure minimum, converting to visual or a circling approach as the situation dictates. This does require a high level of handling skills such as are only afforded by regular practice; however, most airlines cannot afford this, so the 'lowest common denominator SOP' of making every NDB approach follow the 'notional glidepath technique' has been adopted. Fortunately, in companies such as Dan W's and mine, pilots may still descend to the absolute procedure minima, so if that means manually flying a 4-jet level for a mile or so at MDA, then that's what we do - but only because we practice doing it reguarly. But places where the notional glidepath is non-mandatory are, regrettably, becoming fewer.

kriskross 1st Jun 2001 17:38

My Company adds the 50' to ALL non-precision approaches i.e. VOR/LOC/NDB for the reasons stated already many times in the thread. We have twin GPS fitted to our NG 737s, but although Boeing recommended flying non-precision approaches in LNAV the Authority does not yet allow it. With P-RNAV approaching rapidly we are likely to be able to do so in the next few years, and VNAV is likely to be added too!!!
With respect to circling approaches, Salzburg, Grenoble, Nice, Funchal, to name but a few, spring to mind. Not to mention Samos which has a VOR approach at right angles to both runways, but Samos is another story....

Jambo Buana 2nd Jun 2001 19:00

Good thread. To throw in my two pennies worth here are the tolerances as qwoted in the JAR FCL manual for an IR Skill test:

Generally +/-100ft
Starting a GA at DH +50ft/-0ft
MDH / MAP / altitude +50ft/-0ft

JAR FCL 1 Subpart E, 1 E 7.

I think this 50 ft addition is a worthy factor and should be brought in line with the new technology and theories of flying a constant angle descent NPA with EFIS Type machines.
Mind you for 50ft difference on an NPA It would be more valuable teaching pilots to realise the inaccuracy of their altimeters in cold weather Ops.
Boeing are bringing out next year cold weather correction to FMS altitudes on approaches to enable VNAV to be used safely in the colder climes.
Just some useless junk info.

Tartan Giant 2nd Jun 2001 21:35

Thanks again guys.

Your post is good info JB - not junk at all.

Fly safe (when it's cold too !)


frequentboeingflyer 3rd Jun 2001 05:48

Our company flies the B-737-400,500 &700. I have not heard of the 50' correction but I can see it makes sense. But we do correct for temperature and wind to all minimas (In our environment that can be a lot). On our non-precision approach charts we have a table which gives us a stable descent, which gives us a 3 degree profile from MIN to touchdown zone. And flying EFIS we always put our minima at the closest 100 feet above the minima.
And to respond to "wondering"; on the 400 & 500 it is wise to use HDG select on the final intercept due to the lack of IRS ground stations update which we experience frequently at lower altitude. Which again causes frequent map shifts. Of course the non-flying pilot is checking by raw data.
However on the NG -700 map shift seems to be history so flying L-NAV Seems to work fine.

P22 3rd Jun 2001 17:23

One large UK airline flies down to MDA and then commences the go-around, so dipping below MDA. The excuse is that it is written that way in the OPs Manual, which has been approved by the CAA, so it must be OK.

Jambo Buana 3rd Jun 2001 22:00

The NG 737 is approved to operate LNAV approaches these days thanks to GPS being the PRIMARY position input towards the FMC position followed by radio updating and finally IRS information. Makes track keeping really easy on the approach only having to worry about vertical path.
Mind you maybe its become too easy. I was PNF checking a new pilot on line and asked him to shoot a practice NDB approach.
He flew a beautiful approach in LNAV & VS, unfortunately he had all dog ears in the VOR position for the approach, meaning he had no ref whatsoever to the raw data position of the NDB. Lucky one hey?

Tartan Giant 3rd Jun 2001 22:57

Hello there P22,

Can you "cut and paste" the exact phrase/section of the Ops Manual that allows that stupidity please ?

I cannot understand the CAA sitting on their fat arse allowing that !



ManagedNav 4th Jun 2001 02:04

This topic is very interesting...

Our Company has us round-up the published MDA to the next 100 (510' MDA; set it to 600).

I don't agree with the 3deg descent; there are too many variables that can assure you of a missed approach (tailwind will blow your timing out the window).

Why not get on down to MDA and look for the airport until your calculated VDP? Before you say it creates an unstable approach, we have stabilized approach criteria that forbids us from descending faster than 1000fpm under 1000'AGL. Hardly an unsafe situation.....If you don't see it by your VDP then you couldn't make a stabilized approach anyway and it's time to head on to the alternate...

By the way, can anyone fly 3deg slope/700fpm and guarantee that they will be @ MDA prior to the VDP? I imagine that under the right conditions you could very well be looking @ the second half of the runway when you finally do break out...

In summary, don't waste time getting to the MDA, fly until VDP, see runway, establish 700fpm descent, land, buy the F/O dinner....simple as that.....

NorthernSky 5th Jun 2001 08:06

The following link may be of interest:


The CFIT working group concluded (i) that constant descent profiles are significantly safer and (ii) that we must add an increment to minima to allow for sink.

There are lots of ways of achieving a good profile, including groundspeed and time, DME ranges and FIX page information, FPA/FPV for those lucky enough to have it, and last but not least, good flying!

We fly EFIS jets, and never have problems flying these profiles to hit each altitude to an accuracy of +/- 100ft at worst. Usually accuracy gets betterthe further down the approach, and I have never seen anyone reach MDA not in a safe place to land normally.

The working group gives its advice in good heart, in the hopes of saving lives which CFIT might otherwise have claimed.

ManagedNav and BEagle, have you tried flying constant descent profiles? If you have and it didn't work out, I wonder why.... If not, give it a go. I was initially sceptical, but it feels much, much, safer now, and I wouldn't fly level to VDP ever again.

john_tullamarine 6th Jun 2001 04:34

Following on from NorthernSky's comments .. one doesn't need EFIS and other fancy toys to execute a steady descent path angle - albeit that the toys make the cockpit presentation prettier.

All it takes is a quick think about overlaying the desired descent, incorporating reconfiguration a la ILS profiles, on to the letdown plate restrictions so that no requirements are busted.

The normal ILS configuration profile often needs a little modifying, but the end result still maximises the probability that the pilot will detect undesirable trends - which is the big difficulty with the dive and drive school.

Certainly, there will be occasions when the steady approach results in a miss - is that such a big problem ?

Roadtrip 6th Jun 2001 05:54

Circling approaches at wx mins in heavy jets is really not very smart, especially considering the frequency that heavy jet pilots get to practice it -- i.e. almost never. Part 121 heavy jet carriers don't allow circling approaches.

That being said, on non-precision approaches, namly VORs or Localizers, consideration of technique should depend on what the weather conditions are.

If the visibility is poor with a poorly defined ceiling, then a "stabilized" approach probably gives the pilot the best chance of putting the aircraft into a position to execute a normal landing -- i.e. the aircraft arrives at the VDP (or NDP) on speed, in a stabilized descent. The relative deck angle is 3 deg lower than would be on a "dive and drive" approach thus giving the pilots a better downward vision angle for visually acquiring the runway in poor vis conditions. The descent rate is already established. All that remains is to laterally position the aircraft.

If a thick ceiling is well defined and above MDA, and visibility good below, then a "dive and drive" technique gets the pilot visual earlier, allowing a better line up and plenty of time for preparation for descent at the VDP.

It should be noted, however, you should never go below MDA unless the aircraft is in a position to execute a normal glidepath and landing.

411A 6th Jun 2001 11:28

Have always thought that the "dive & drive" method (within reason) was the best, and the need to add 50 feet to the MDA is a complete waste of time IMHO. To go-around without descending below minimums is not all that difficult with all engines, more difficult with an engine out.

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