[jhb_airlines] Re: Another one for Bones...

  • From: "Bones" <bones@xxxxxxx>
  • To: <jhb_airlines@xxxxxxxxxxxxx>
  • Date: Thu, 24 Nov 2005 11:43:21 -0000

Having had a good look at the plate now I honestly think this NDB procedure
is one of the least friendly I have come across. The standard overhead
procedure is normal enough but those two DME arc joins are not at all user
friendly. Too much work to do in a short time and too much cross checking.

I'll run through the direct arrival from the northwest as this seems to be
the one you tried to fly.

Assuming you are tracking in on the 135 QDM to the NDB at 2800ft all is
plain sailing until you get to the 10 DME point. Note that this is the IAF
and you need to buttonhole it quite accurately - if you are off the fix by
any margin it makes the rest of the procedure a real handful.

At 10 DME you start the procedure. This calls for a right turn onto about
225 to pick up the 8 DME arc. At the same time you need to start your
descent to 2200 ft so you have several things going on at the same time - a
descent profile and trying to pick up the correct arc. I don't like this as
it divides the concentration but I confess I don't like arc procedures
anyway.

Assuming you hit the arc correctly and at the right height you then have to
watch out for the intercept radial. With a normal ADF (RBI) you do indeed
have to look for the 14 degree split but with an RMI always giving you a
true QDM you wait until the needle is on the 108 bearing. At this point the
workload increases. You start your turn to intercept the 094 QDM and also
start your descent to 1650ft. If you have got inside the 8 DME arc then
things will happen faster and so you must be prepared to descend quicker and
turn quicker too. OTOH the procedure is designed around a nominal approach
speed of (I think) 130kts so if you are faster or slow you have to adjust
your turn onto final accordingly to avoid an undershoot/overshoot.

The critical point in this approach is the 5 DME check as it is the first
point at which you can check you are correctly on the centreline at the
right height. If things are not good here then it really would be better to
throw it away and try again. At 5 DME you then need to descend to the SDF
height of 570ft. It doesn't mean you have to be at 570ft by the SDF at 3 DME
but you can't go lower than 570ft at this point. The recommended profile of
305ft/nm is a standard 3 degree glidepath so you can settle on 5 x G/S for
your RoD.

Assuming you are nicely on the centreline and don't have to worry about
descent rates as well as course corrections you should be at 1340ft at 4DME,
1040 at 3DME, 730 at 2DME and 410ft at 1DME (or your minima for that
approach). MAPt is 1DME so if you can't see the runway it's a go around.

If you do see the runway it will be to one side (left) of your approach as
the IAP is offset by 6.5 degrees from the runway centreline. You don't hit
the runway centreline until 0.8DME - by which time you'll be setting up for
landing or going around anyway..

bones

-----Original Message-----
From: jhb_airlines-bounce@xxxxxxxxxxxxx
[mailto:jhb_airlines-bounce@xxxxxxxxxxxxx] On Behalf Of Peter Dodds
Sent: 23 November 2005 12:05
To: jhb_airlines@xxxxxxxxxxxxx
Cc: pdodds@xxxxxxxxxxxx
Subject: [jhb_airlines] Another one for Bones...


Last night I was flying Garry Blaisdells Twin Otter into
Gloucestershire with VATSIM, and with the poor weather was
obliged to fly the NDB/DME procedure to 09.  Whilst in flight, I
downloaded the plate and with little time to study it, although I
pegged the DME arc inbound from the north, I botched the final
turn and trying to get back to the correct QDM, missed the final
aproach fix at 3DME.  (It seems to be called the SDF now, not
FAF - what is SDF?). I was able to complete the approach visually
fortunately, because, although viz was only just about 3 miles,
FS obligingly shows the PAPI lights through the fog/mist!

The ADF instrument in the Twin Otter has its compass bezel slaved
to the HSI, making it a Radio Magnetic Indicator (RMI) I believe,
not an ADF. Now I think that this affects the instrument
"picture" you look for at the point where you start the final
turn at QDM108degrees/8DME. So what threw me last night was that
I had forgotten that the ADF needle remains horizontal in the DME
arc, its the bezel that turns - right up to the moment of the
final turn. What I was looking for was the needle pointing 14
degrees "up" from horizontal to turn from a QDM of 108 to the
final approach QDM of 094, when in fact I think should have
turned when the instrument bezel had turned so that the still
horizontal needle was pointing to 108 degrees.

John, could you do one of your excellent tutorials on flying this
procedure?  Am I right?  I tried it again this morning and got it
roughly right, but when I came out of cloud at 1600 feet, I was
slightly off line.

Peter



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