Good and bad NDBs

[tmmorris]

Yesterday I decided to do some NDB tracking in anger - as I usually use GPS (don't start that thread again!). Part of the aim was to see just how far off the desired track the NDB actually left me, as well as to revise all that "+15/-15" stuff...

Anyway, I was struck by how much better the Brize NDB was than the others on my route (OX and GST). The GST one is of course notorious, particularly to the east (as I was) because of the hills. But the land around Kidlington is reasonably flat, yet I was still 5-10 degrees out consistently. Whereas the BZ beacon was virtually bang on for 15 miles in either direction - never more than a couple of degrees out.

So: is the BZ NDB a better one, or does it all come down to terrain? Or is it just more powerful?

Tim

[S-Works]

what type of receiver do you have? I have an old KR85 which has 2 sets of chips one for the high end and one for the low end. When it goes it of tune I can get some beacons like the needle is glued and others are miles off even when on top of them. A good example is Cranfield 850 and Wescott 350. I could be sat on the runway at Cranfield and not get the CIT yet get a perfect lock on WCO. A week in the avionics shop and it fine for another year!

[IO540]

I think a lot depends on what is around, in terms of high ground or water, and how symmetrically these ground features are located relative to the track being flown.

An hour ago I did the Padova (LIPU) NDB Lctr DME approach (one of the more complicated procedures around; the IR instructor would have loved it) and the 014 NDB inbound places you well off the track. This was obvious on the GPS (which was used to fly the STAR as well as the inbound) but if all one had was the NDB then one would have ended up in a position from which a visual approach would be tricky.

Same as Shoreham 20 then :)

I think NDB procedures really exist to enable one to fly a GPS overlay approach officially....... but none of the retired RAF navigators in the CAA is going to acknowledge that :)

[tmmorris]

I hadn't thought about the equipment issues, actually - it's not my aircraft. I'll talk to the owner and see if he finds the same.

Tim

[helicopter-redeye]

What was the wind?

[tmmorris]

Roughly southwesterly - I seem to recall about 230/25. Why?

Tim

[S-Works]

I suspect he is intimating your problem was wind drift rather than equipment.

I rather assumed that you knew the concept of ADF tracking rather than homing?

[High Wing Drifter]

I've been using GST a lot recently and certainly no hint of it being particuarly inaccurate to any noticable degree upon arrival from FL90. Of course, height could be a factor as MF bands are subject to some refraction and reflection and the closer you are to the surface the more affected you will be by these effects where they occur.

Apart from that there is just the usual issues of being within the DOC (25nm for GST and OX I think), wings level, etc.

[Dan Winterland]

The NDB is a simple animal - it sits in one place and transmits. Therefore, most errors you notice are more likely going to be receiver errors. I know that you can get coastal and/or terrain refraction, night error, dip and the ADF's habit of pointing at thunderstorms - but in theory, the receiver makes all the difference.

Of course, your receiver is going to be better at pointing at a stronger signal. I don't have the published protected ranges of the three beacons but I do remember from flying at Brize for may years that the BZ is quite powerful. And considering that the owners of the BZ have considerably more resources that the owners of OX and GST, there may be something in this too.

But with all these errors, we have to ask why in this day and age of GPS updated RNAV we are using a navaid designed in the 1920's. The procedures still exist and sometimes we have to fly them, so IO540s comment reagrding the NDB procedure being used with a GPS overlay is very valid. Last week, I flew into an airfield in China which like all airfields in this region is undergoing major WIP due to the fact that China's aviation market seems to double in size every two weeks. The airport managed to have both ILSs out of service at the same - with the VOR approach being reported as unreliable! An NDB was the only option. But as the airfield is in a steep valley and it was night and there were CBs around, there was no way we would have continued if the GPS hadn't been reporting 'accuracy high'.

[tmmorris]

I wondered if that's what you meant, helicopter-redeye, in which case you insult my intelligence... or at least the training received by IMC pilots. I do understand and use the methods of correcting for drift with an ADF, hence my comments about +15/-15 - I learned a lot from 'VOR, ADF and RMI', which is a great book. What I said was that even when that method showed me on track, the GPS (and the view out of the window, when I could see it, which was about 30% of the time) nevertheless showed that I was 5-10 degrees off track.

I totally agree with Dan about equipment, though. I used to fly a Cherokee 180 with the original ADF - receiver aerial was a wire running from the top of the fin to the cockpit - and it was unbelievably sensitive and accurate compared to the crap fitted in current aircraft. It was an ADF from the days when you really, really needed a good one. It had a rotary tuning dial - someone had fitted a slaved digital frequency readout as well, for ease of use - and I could get the OX from 30-40 miles away.

Tim

[IO540]

Most NDB errors come from the radiation pattern getting bent, not from the receiver being wrong.

I have a dual-needle RMI; one needle for the ADF and one for the VOR. Flying across large expanses of flat totally boring terrain (say N France) in clear weather, the ADF is spot on, within a degree or two. But any assymetry on the ground, and this assymetry can be miles away from the NDB, and the radiation pattern changes. It doesn't matter what the receiver does; it cannot detect the distortion. There is no way to detect it. The pattern is bent before it is received.

The other thing is that an ADF is meant to be accurate only on the four cardinal bearings. In between you can get a significant error. So if you are flying on say a 360 track, and there is an NDB at your 045, it could actually be at your 055 or whatever. But the system should be accurate if you are flying directly to/from it, or exactly abeam. I don't think there is any spec on the max allowed error if away from the cardinal bearings.

I have a KR87, BTW, driving a KI229.

It's common for the NDB/ADF system to be off by 30 degrees (yes 30) due to terrain assymetry, and the error will change according to how close you are. From say 30nm it might be spot on. When you are at 8nm it might be 30 degrees off. Then when you are at 3nm it might be correct again.

NDBs are OK for navigating across vast expanses of land or sea, and are accurate for that (thunderstorms excepted). They are also very accurate as locators; flying overhead. They are much cheaper than VORs to build and maintain.

The #1 State Secret of the anti-GPS lobby is that they aren't any good for most instrument approaches - the very place where you need to be in the right place....

The most hilarious thing is that you have to carry an ADF for IFR in CAS, in UK airspace.

[BEagle]

It sounds as though the loop element of your ADF antenna is tits up - you should be getting ±3º with that receiver system.

Personally I used to put the inbound NDB course on the HSI beam bar with a blended Y-code GPS and LINS driving it - but that wasn't IFR approved, so it could only be used as a back up to the stone age ADF.....which was :ugh:!

[Dan Winterland]

Hi BEagle,

I'm not sure many readers of this forum will have access to Y channel GPS!

But it is time we left the middle ages WRTo NAVAIDS. My company have been cleared for Fully Managed Non Precision Approaches (GPS as well!) using a Nav system you will be familiar with. Watching the aircraft fly a perfect 3D NDB approach profile on autopilot is a thing of beauty, especially when the weather is bad, it's night and there's high ground nearby - and it's your third sector of the day. But this is using an FMC position from a triple IRS mix with GPS update. Still, we had to have the NDB needle selected on the Nav Display as we are using the NDB Jepp plate for reference! Although last week when flying into Guilin, I think the it was poining at a CB the one time I glanced at it. If ever faced with having to do an real NBD approach in those circumstances - it's time to divert!

[BEagle]

Quite so, Dan!

The folk I work with now aren't even allowed to fly non-HSI approaches! So if the aid cannot be selected to the HSI, you cannot fly the approach!!

All those NDB, TACAN and the odd VOR approach we used to fly on the basic RMI would be forbidden....:rolleyes:

Watching the aeroplane fly its approaches, automatic level offs etc made me realise how prehistoric the old Vickers FunBus really is! But the A320/A330 are even better...:ooh:

[411A]

>>All those NDB, TACAN and the odd VOR approach we used to fly on the basic RMI would be forbidden....<<

And therein goes the proficiency.
I recall years ago flying across Bulgaria in a 707, and was doing some manual loop/180 degree ambiguity bearings on a couple of NDB's along the route.
The First Officer had no idea about any of this, but the navigator we had along (positioning) began to explain...and the F/O's eyes started to glaze over...:}

[BEagle]

Too true, 411A!

I'm keen to see GPS receive greater acceptance in the UK and it's pleasing to see the CAA making cautious progress at last. I guess the mythical 'ex-RAF navigators' are keeping themselves happy shooting astro from the roof of the Belgrano (CAA building at Gatport Airwick), whilst the young(-ish) thrusters convince them that GPS isn't just some Devil's device!

When ADF replaced MFDF, people thought it was fantastic. As they did when the 4-course MF radio range system came into use in the US in 1929 (I guess even you weren't 'flying the range' back then, 411?) - and again when VOR came along. LORAN-C was another quantum leap, as was Omega....

So why is GPS viewed with such suspicion?

I went flying with a qualified (but lapsed) pilot today in an aircraft fitted with a panel-mounted GPS. She told me that none of her previous instructors had even bothered to switch the GPS on.....

WHY?? I spent a lot of money equipping all our aircraft with Garmin GPS150 or GNC250 systems, yet the Jurassica cannot be bothered to learn how to use them......and it's so VERY simple!

[Dan Winterland]

And therein goes the proficiency.

But the safety has improved. You can argue that reduced profiency reduces safety, but the risks of flying such a procedure are high. Well, high by today's standards. Better to manage the risk earlier in my opinion.

My previous aircraft was a wide body classic jet. Our IR renewals in the sim had us flying an NDB approach using the RMI. For real, I wouldn't even consider doing it without the triple mix IRS with GPS update info on the HSI. As it was, I never had to. Our chief pilot mentioned that he didn't really want anyone flying them for real either!

Now on the electric wonder jet, it really is no fuss. And with a EGPWS overlay on the ND there is no more risk than flying an ILS.

[411A]

I would respectfully suggest, Dan, that perhaps your chief pilot does not really properly know how to actually do the NDB procedure.
Having done a LOT in heavy jets, it really isn't that hard...if properly performed.
The operative word is (of course)...properly.
In this case, if he does not...proficiency down the drain.
What an absolute shame.:uhoh:
Having said this, when I did my original training at PanAmerican in the 'ole B707 (yes, a rather heavy jet at the time) the FAA Inspector mentioned...'I want to see an NDB approach on two engines', which was promptly accomplished, by yours truly...two engines out on one side.
Properly.
I can assure you, they did NOT give these ratings away, without the pilot concerned actually demonstrating the proficiency required.
It all comes down to...train hard, fight easy.

PS:
What do you do, Dan, when you are dispatched to a distination that has ONLY an NDB approach to one (or perhaps two) runways?
Cancel?
Call in sick?
What?

[Piltdown Man]

Remember that an NDB is a non-precision navaid and the minima that are used are appropriate to that. But the clue is "non-precision", ie. It is not very accurate. So, by comparision with a dead accurate, fully updated GPS it's performance might appear to be woeful, but it does do what it says in the tin. Unfortunately, Mr Bush has a veto on your GPS approach so I'll stay in the stoneage untill that changes.

PM.