View Full Version : ndb approach map and mda


winggeek
11th Mar 2006, 17:42
hi all have a question for ya's,

lets say your flying along ifr from dublin to shannon and in your cruise you get an engine failure so you decide to land at a suitable airoprt, after checking the atis but it is at mins and for arguments sake this airport has no ils and only a ndb approach. so you get safley into the approach do you

a) fly the approach hit mins then fly level at mda in the hope you get better ground visual reference between your level off and the miss approach point, and lets say somewhere along that segment you do get a better ground visual and decide either to go straight in or do a circle to land.

or

b) fly the approach you hit mins and start missed approach staright away,


now lets say you have two perfectly good engines and your doing a iaa me ir flight test and your doing a ndb approach and you hit mins wat would you do in this case a or b? and wat would be the case if your doing a one engine out ndb approach for you ir me?

yet another question

if the descent part of the approach starts at 8 dme but is only a shallow descent from 2000ft to 1200ft at 3 dme, would you

a) do a shallow descent from 8 dme to 3 dme followed by a 500fpm descent

or

b) wait until 5.5 dme and keep a constant 500fpm descent


cheers lads

i know there is a lot of ifs and buts that can be said for this example and there is queit a few question to be answered, but thanks to all who answer,

AND NO ONE IS SUGGESTING DIVING IN!!

wingeek:ok:



Pilot Pete
11th Mar 2006, 18:29
I would check the ATIS before deciding where to divert to, not once committed to an approach. I would divert to the nearest suitable, not necessarily the nearest, which might be an airfield a little further away with an ILS with the weather you mention...

As for constant descent then missed approach or fly level at MDA, do what your IR instructor tells you to do. In my company (big jets) we fly the constant descent and then do a G/A at MDA +50ft as flying level at MDA and then making a dirty dive if you get the lights is guaranteeing an unstable approach. Can be done in light aircraft, but can still lead to some pretty unstable diving as the lights appear just infront and 700ft below!!

So do as trained to pass your IR and then do as your company advises once you are employed.

PP

winggeek
11th Mar 2006, 18:36
cheers pp,

i know wat u mean about the atis and most suitable field.... for arguments sake lets say all these ended up leading you to a ndb approach etc i have been instructed by 2 instructors one saying go level to map and the other says missed approach straight away, they both have good reasons, so this is why i have asked the question, cheers for the answer pp

winggek

anyone else have a different opinion???

TRISTAR1
11th Mar 2006, 18:51
Pilot Pete is right.

Just about all operators fly a stabilised nominal 3 degree approach.

Dive and drive was got rid of many years ago.

Keygrip
11th Mar 2006, 19:15
I don't think he's suggesting that.

Consider he gets a very stable approach - either multi engine, single engine or asymmetric - matters not. Stable approach.

Gets to MDA but is still a way from the threshold and slant vis does not allow sight of approach lights, threshold, whatever.

One of his instructors says immediate go around, the other instructor says fly level at, or up to 50 feet above, MDA until the MAP. IF DURING THIS LEVEL SEGMENT he gets the required visual reference(s) THEN he makes the decision to continue THAT approach in a stable manner OR carry out a visual circle to land (still stable).

Nobody has suggested diving to the surface in either of his questions.

I think the second question is more along the lines of - theres a required "altitude at distance" point of, in his example, 1200 feet agl at 3 dme. The approach plate shows the FAF as 8dme - but a 500 fpm constant descent will not get him to 1200 at the same time as getting to 3dme.

So, does he (a) delay TOD until a 500fpm WOULD get him to 1200@<hidden>, (b) decsend at a lower fpm to choreograph arriving at 1200'/3dme, or (c) start the 500fpm at FAF, then level off at 1200 and fly level to 3dme before starting a second, stable, 500fpm, descent (presumably 3° slope on the IR flight test as suggested).

No diving - we all know that's a bad thing.

Delay TOD, descend gently, or level off before 3dme and descend again?

winggeek
11th Mar 2006, 19:26
thats exactly wat i ment, and no one is diving in!!!

cheers keygrip :ok:

TolTol
11th Mar 2006, 20:12
Well here is my thoughts and what I've been instructed to do:

We start the decent at 5.5d and maintain a constant gradient down to MDA. The reason we start at 5.5d and not 8d is so that we have a constant decent rate down to mda (which helps during asymmetric approach). When we get to the mda we level off until reaching the MAP. However! The MAP is at the NDB and the NDB is half way down the runway, so there is no point in waiting for the MAP so we go around at 1d (this is approved by the IAA for our school). We aim to be at mda at 1d.

1800-how'smyflying
12th Mar 2006, 01:24
OK Winggeek as promised here is my reply, hope you can understand it.
The way I was instructed to do an NDB approach was to get to the TOD, and immediatly descend at 1000 FPM to your MDA (but check there is no MIN heights before your MDA on the plate).

So if TOD is at 8 DME, and MDA is 400ft at 2 DME, just 'buzz on down' to 400ft. As there is no "not below" altitudes it is safe* (*although I can't guarentee safety).

The logic behind this method is that, because on an NDB approach the signal recieved is not totally accurate, and the closer we get to the ground system the less accurate it becomes, we are constantly changing heading.

These heading changes may become greater and more frequent as we get closer to the ground system and our MDA.

So in a high stress situation, as you approach your MDA and realise you are 4 degrees off your track on your NDB and about to partial your IR, you put all your attention into fixing your heading and bust your MDA.

Descending to your MDA straight away eliminates this risk.

Ok, well I hope I worded that well enough for people to understand what I mean. I don't know about others but I didn't feel comfortable with this even with the logical explaination, and my instructor didn't force it upon me. But then again I partialed!

Its like the whole "gear up, flap up" -vs- "flap up, gear up" thing, different schools and different instructors have different ideas. Either way I'm sure you will pass.

paco
12th Mar 2006, 02:12
I think scenario a is the appropriate one. You have a problem - are you going to stooge around all day and wait for the other engine to quit? I've never understood this on IR tests - an engine quits and you immediatley go into the hold for 15 minutes! You want the shortest and safest way down, and if you have to do a procedure, so be it, but there may be a more suitable place to go that has radar - you don't necessarily have to go for the nearest airfield.

In Canada, where they have a more practical approach, my examiner gradually socked out all the alternates until I had no alternative but to say I was going to bust minimums to get in - a better alternative to running out of fuel in the air. Perhaps your examiner was trying to bring a bit of the real world into your test.

Anyhow, if it helps anyone, here is what I teach about approaches:

An approach, especially a non-precision one, is characterised by large "steps", that is, major descents at certain stages, requiring large power changes that can be a pain with an engine out, so it's often a good idea to keep a consistent glide path as much as possible, remembering that the minimum heights at each step are just that - minimum heights. There's nothing to stop you being above them if you are actually descending under control. In other words, MDA (or any altitude, for that matter) is a height below which you must not descend, and not (necessarily) a height at which you must fly for the procedure, so you don't have to go to it immediately and make work for yourself in the final stages if you have a problem. In other words, it is a limit and not a target. My own preference is to descend down to around 100 feet above, and nudge my way down.

The idea on a non-precision approach is to fly level at MDA once you reach it after passing the FAF (assuming your heading is more or less in line with the runway) until the time you expect to be over the threshold (if you are not in line with the runway, you can expect a circling procedure, of which more later). Because timing is so important, it follows that your speed control should be precise.

Tip: Check the compass deviation - a couple of degrees means a couple of miles error!

You may obviously go down further if you can see where you're going (that is, you have established visual reference), but this should be at normal touchdown speed. It is a good idea to get down quickly enough to allow yourself at least a short time in level flight - ideally, at the visibility limits, especially as the MAP is often past the point from where a safe touchdown can be made. If you can't see anything by your estimated time of reaching the threshold, you must go around (more technically, if you don't have visual reference at the MAP, you must start the Missed Approach Procedure).

The reason why you don't do a dirty dart is that there are no performance figures for a go around in the landing configuration - you are taking quite a chance, and it is quite an urgent procedure - you have just tried to get as close to the ground as the height of a small apartment block, which is a pretty fine tolerance, given the velocities involved, and you don't want to be there if you can't see where you're going, so the routine is to add full power, then get the flaps and gear up. You won't gain much by pulling the gear up first at that speed, because it is producing minimal drag and will do until you are going much faster.

Drag varies as the square of the velocity, meaning that if you double the airspeed, you get four times as much. Conversely, reduce it by half and you get a quarter of the drag. Thus, at low speeds (as when landing), the drag caused by the gear has more or less the same effect within a small range or, at least, the incremental effect is less (review parasite vs induced drag). Put another way, the drag doesn't start to vary much till about 20 kts above your landing speed. For example, pulling the gear up at 60 kts may mean you won't climb at all. Doing it at 70 (+10 kts) could mean 50 feet per minute, 80 (+ 20 kts) could give you 100 feet per minute. So far so good, but 90 kts means 200 fpm, and so on.

The point is that, only from about 30 kts or so above your landing speed will you get any real benefit from raising your gear and expecting the decrease in drag to be the same as an increase in thrust.

Check it out with your own machine at a few thousand feet. Trim out in the landing configuration straight and level, apply full power and note the vertical speed (it should be a climb at this point). Then check the figures for every 10 kts above the original speed. You will find that, after about 30 kts, the rate of climb will reduce (after a modest increase), and your machine will even start to descend after about 60 kts! Airspeed isn't always the answer when you've got a lot to do in a hurry and you need to establish some priorities. Of course, you should not stay at such a low speed for long, as you need to get to normal climb speed, but at least you're getting further away from the ground. The point is, you can fly first and clean up later.

Phil

scroggs
12th Mar 2006, 08:45
There's a lot of technical explanation of various techniques here, but I think it can be boiled down to a couple of points.

1. A stable, constant-angle approach is safer, and is standard procedure for all large-aircraft operators..

2. There is little to be gained from flying level at MDA to the MAP - and it's potentially very dangerous if you decide to dive for the runway once you see it. Bear in mind you have visibility/RVR limits for a reason!

3. Decide what you're going to do, brief it and then fly it. Don't make your plan up as you go along, and don't change a good plan.

Scroggs

moggiee
12th Mar 2006, 21:23
Dive and drive was got rid of many years ago.
What a pity that the North American continent seems less willing to take this very sensible move!

moggiee
12th Mar 2006, 21:27
One of his instructors says immediate go around, the other instructor says fly level at, or up to 50 feet above, MDA until the MAP. IF DURING THIS LEVEL SEGMENT he gets the required visual reference(s) THEN he makes the decision to continue THAT approach in a stable manner OR carry out a visual circle to land (still stable).

Either "approach" is correct - you may go around at any time between commencing the approach and reaching the MAPt. Provided that you never go below MDA or outside of the prescribed +-5 limits on tracking you are OK.

However, this important issue of standardisation should be addressed to the instructors in question and or CFI/HoT.

moggiee
12th Mar 2006, 21:33
The way I was instructed to do an NDB approach was to get to the TOD, and immediatly descend at 1000 FPM to your MDA (but check there is no MIN heights before your MDA on the plate).
So if TOD is at 8 DME, and MDA is 400ft at 2 DME, just 'buzz on down' to 400ft. As there is no "not below" altitudes it is safe* (*although I can't guarentee safety).

A clear recipe for disaster. Once you level at MDA many miles from the airfield, you then drag in at low altitude, low speed, high drag, high power but most imortantly with a restricted view due to the unusually high nose attitude (3 higher than it would be for a stabilised approach).

This restricited view in marginal conditions frequently leads to a subtle and often unobserved entry into a descent - a factor that crops up in all too many accidents.