NDB approach tolerances

[bogbeagle]

I quite often find that RMI equipment as fitted to my club' PA 28 is in error by about 10degrees, when compared with the ILS or the final approach track as defined by radar.

In the final stages of the approach, even to minima, a 10 degree error is not a huge problem as the aircraft can be manoeuvred to a landing. However, what is the "fat" that is designed into the approach. How much tolerance do approach designers build into the NDB procedures? I am, of course, concerned with obstacle clearance, primarily.

Bogbeagle

[bookworm]

While you may find the ATCOs here have the answers to your question, if you ask it on Tech Log you'll find the depth of knowledge on PANS-OPS Vol II, which is the relevant document, quite impressive.

[chevvron]

The Terminal Procedures designers at the Directorate of Airspace Policy (DAP)use templates showing 'worst case' tolerances when designing any type of instrument approach procedure.

[spekesoftly]

Some of the 'worst cases' are Training flights going-around from an NDB approach, straight into the teeth of down-wind traffic in the visual circuit! :uhoh:

[phallic]

NDB is Class Bravo +/- 5 degrees in tolerance. Worst case scenario will be taken into account in the procedures design and a/c should be within MSA during the final approach. As said above the most likely hazard is collision with downwind circuit traffic.

HTH.

[Pierre Argh]

BB

I'm not sure if you're talking about accuracy of the kit in your aircraft or that on the ground? Rest assured, however, that the dimensions of the final approach segement (the area considered for obstacles when determining the approach minima) is considerably larger for a non-precision approach (like NDB) than that for a precision approach like ILS... particularly at the FAF end of the segment (where you might say accuracy is most relevant). This probably takes into account the 10 degree difference you are experiencing and more.

Another factor that you might want to consider is that the FAT has to be aligned through the NDB (as that is the only means of obtaining course guidance) and yet pass within a given distance from the runway threshold (IIRC within 0.5nms of THR?). So the FAT for such an approach may not be aligned with centreline, deliberately.

Remember the aim of a non-precision approach is that you are able to see the airfield by the time reach the FAF... whereas on a precision approach you should be able to see the runway and have a much better chance (I can't remember the percentages, but they are quoted) of landing from the approach?

Hope that makes sense?

[Standard Noise]

I'd say we're very tolerant when it comes to NDB approaches at Bristol when you consider the state of some of them!:}

[bogbeagle]

Pierre Aargh

I'll re-phrase my question.

During my NDB approach I have descended to MDA and am inbound on what I believe to be the FAT, by reference to my RMI.

How many degrees may I stray from my FAT before I am at risk of clouting an obstacle?

I will only continue my approach whilst I remain with 5 degrees of the published FAT, but errors within my ADF, as fitted, could mean that I am 15 degrees from the ideal track. I was wondering what area is considered when designing NDB approaches and determining the OCH?

Put simply, what level of inaccuracy can be tolerated on an ADF before it becomes unfit for an NDB approach?

Bogbeagle

[Honey Monster]

Bogbeagle

The NDB transmitter may be anywhere on the airfield (ditto for VORs). The ILS localiser is necessarily sited at the upwind end of the runway to provide accurate centreline guidance.

As you approach the runway on the centreline, the ILS localiser needle should be in the middle whilst the RMI needle will be pointing towards the NDB transmitter.

If you fly the NDB procedure, assuming the cloud ceiling is high enough, you will break cloud at or above MDH in a position where you can see the runway and visually position the aircraft on final approach. The procedure is designed to keep you clear of obstructions.

[DFC]

The area taken into account for obstacles is 2.5nm wide at the ndb and splays at an angle of 10.3 deg as one proceeds away from the ndb.

The inner 50% of the area is the primary area where you get the full obstacle clearance and in the outer parts the clearance reduces to zero at the outer edge.

Thus with an NDB on the field, you would have to be quite a way off to hit something at the FAF.

Similarly, with an NDB some distance from the field, if you were at the limits of the obstacle clearance, you would find it hard to continue with a straight-in approach.

Fly the NDB as accurately as possible unsing the ADF and with GPS recording where you go by following the ADF. After you land, upload the GPS track to Memorymap or Google earth and draw in the obstacle clearance area described above to see just how well you did.

Remember that with an on-airfield NDB, the approach track will always cross the runway centerline at an angle and will never follow the final approach track to the runway used by radar or GPS unless the NDB is on the runway centerline.

Regards,

DFC

[Pierre Argh]

Bogbeagle... as you may have guessed the design of the procedure assumes a level of accuracy from the pilot, and although that accuracy will be included within the original designation of the Final Approach Segment Size... the specifics are not available to a procedure designer... and I believe you should be asking that to a pilot Instrument Rating examiner not an ATCO/Procedure Designer.

What I can assure you is that, as DFC says, the segement takes into account aid accuracy and error, pilot accuracy (whatever that is) a whole host of other factors and then a bit for the wife & kids for good measure. Look at the figuyres above, the the segments for NP Approaches are massive... hence high minimas often have to be accepted.

[NorthSouth]

Remember the aim of a non-precision approach is that you are able to see the airfield by the time reach the FAFI'm sure that's not what you meant. The FAF (if there is one) is always several miles from the airfield and it's the point at which you commence descent on the final approach. If you aimed to be visual when you got there you might as well fly VFR.
Also, it's not true to say that in non-precision approaches the aim is to see the AIRFIELD while in precision approaches the aim is to see the RUNWAY. Most NPAs are aligned with runways and that (or the approach/runway lights) is what you aim to see. Aiming to achieve visual reference with the airfield only applies for aerodrome approaches i.e. those not aligned with a runway.
NS

[yellowplane]

similar but not the same... old hand perhaps out ther for this one..

We work to 60 degrees outbound for the drop and 10 degrees inbound. Can't actually find anything in the books but is there anything different that anyone knows about?

[Pierre Argh]

NorthSouth

my mstake... you're absolutely correct, I meant MAP not FAF, I guess I was distracted by talking about Final Approach Segments and get it arse-about face. Thanks for pointing it out!

However, whilst Non-precision approaches tend to be aligned with the runway (for conveniece as much as anyhting, and to give the poor guys up front the best chance of landing)... the aim of such an approach is, however, only to allow the pilot to become visual with the airfield so that they can position themselves to land... not to be fully set up to land as in a Px approach (although most Non Px approaches are good enough these days that i can see where you might be confused?)