[jhb_airlines] Re: Article

  • From: "Bones" <bones@xxxxxxx>
  • To: <jhb_airlines@xxxxxxxxxxxxx>
  • Date: Wed, 24 Nov 2004 05:10:04 -0000

I honestly don't know what the cockpit is from on that first photo but I
have a sneaking suspicion it's the much modified NASA Boeing 737.

Yep, MCP is indeed Master Control Panel. You can take this as the block =
switches on the default FS aircraft covering the HDG, ALT, Speed, VS, =
BC buttons etc or as the Go Flight MCP unit. As most add on aircraft =
have a
similar bank covering these functions - and the article was beginning to
dive into slightly deeper waters - it was necessary to start using more
exact terminology to avoid generating confusion.

Speed control
Speed behaviour in FS is pretty accurate and you hit the same problem =
in the real world. In fact slowing down in the descent becomes a problem
even down to GA levels once you get a fairly streamlined aircraft. A
Cherokee or C172 does not have any problem but move up to the PA24 =
or PA31 Navajo and you get a shock.

The basic dilemma is that sleek aircraft cannot descend and slow down at =
same time. Even with much reduced throttle many aircraft remain close to
cruise speed when descending and so you are in the trap of foregoing one =
the other - it is either rate of descent or airspeed that you opt for. =
can't do both.

The trick is to ignore the book figures for descent rate for the =
and use something less. In the PA31 we used a 500 fpm descent rate at =
as this is easy to calculate. If up at FL100 it would take 20 min to =
to sea level and at 3nm per minute this would mean starting down some =
from destination. Also, at 500fpm, we had a bit of flexibility as we =
still slow down a bit or we could increase ROD.

For the bigger jets I'd ignore the default 1800 fpm rate set by the
autopilot and use 1500 fpm. You would have to check each aircraft to see
what ROD they use so the above is just a guideline. At the shallower =
rate the autothrottle would probably have a bit of power on and so you'd
have a small degree of control over speed settings. An example is one of =
default aircraft (747 or 737 - I can't recall) that won't peg at 250 kts
during normal autopilot descent but stays up nearer 270/280kts. Reducing =
will bring the aircraft back under the autothrottle envelope.

Admittedly this will entail a longer descent profile and so you should =
for this. Alternatively use FSNav to compute the top of descent - but =
sure you change the ROD in the aircraft's profile first.=20

Speed brake is the ultimate option and should be used if ATC ask for an
expedited descent or if you know you've miscalculated the bottom of =
point. Flaps are generally no help but again this varies with aircraft =
- I think the 747 has a high figure of 270kts for first stage setting =
there are probably others too.

I've had a look at both the PMDG 737 and IFDG A320 air files and it is
possible the spoiler drag is in Section 1101 <36h>. I'd play with this
carefully though as the former only has a value of 15 in that box =
the latter has 123! The default 737 is even higher at 261..

Missed Approach Procedure and Decision Height/Minimum Descent Height
These used to be shown on AIP approach plates a while back but they have
been removed as they are now considered company minima. The Jepp/Aerad
charts still give the data (airlines then factor this for local use) but =
don't have to resort to searching these out. I will explain..

A few years ago the CAA was criticised for not stipulating minima =
this allowed non company aircraft (business or private aircraft) to =
in far worse conditions than the airlines or charter aircraft (which =
legally bound to publish minima for their pilots). A lot of other =
had already produced State Operating Minima and the CAA eventually did =

To dig this out requires a bit more digging into the AIP. Ignore the
approach charts and open up the Textual Data page for the airfield. =
down to Section 2.22 Flight Procedures and look for sub section 6 -
Aerodrome Operating Minima for Non Public Transport Flights. This should
give you the basic data you can go on.

For example at EGNS it gives for the ILS/DME 26 approach a DH of 250ft =
minima of 700m RVR. If the glidepath is U/S and you are flying the
Localiser/DME approach the DH is raised to 400ft and RVR to 1200m. A =
approach (SRA) has a much higher limit - 600ft - because it is deemed a =
precision approach, as are NDB or VOR letdowns. If you look at Heathrow =
almost all other airports) the limits are much the same - 250ft for an =
400ft for a Loc/DME and 600ft for most others.

The Missed Approach Point is shown on the approach plate but not for an =
approach as it is redundant. This goes back to definitions so bear with =
whilst I explain again.

An ILS is defined as a Precision Approach. Because of this the =
is slightly different in that the point at which the approach is =
is called the Decision Height. Non Precision Approaches (VOR, NDB and
Localiser only) use a different point called the Minimum Descent Point.
There are subtle differences.

With the ILS the aircraft is allowed to descend to the DH (usually 250 =
and, if the crew don't see the runway or approach lights they HAVE to
execute a missed approach. In other words there isn't a MAPt for this
procedure because it is the same point as the DH.

For Non Precision Approaches the aircraft can fly down to the MDH but =
it is allowed to level off and remain at the MDH until it gets to the =
MAPt -
only then does it go around. So, a MAPt (I'm adding the little T because =
charts now show it as MAPt rather than MAP) is only necessary and shown =
non precision approaches.

If you are wondering about the oddity of the non precision approach then =
me explain. In the old days prior to DME a VOR or NDB approach was flown =
timings only - pilots didn't have an distance information to show how =
they were to touchdown. This still applies to airfields without DME too =
- it
isn't a historical issue (look up the NDB plate for Cranfield).=20

The approach procedure was based on flying over the beacon at a set =
and flying outbound for several minutes - the exact timing depending on =
approach speed. You then turned inbound and descended to the MDH. If you =
it right this would almost be like an ILS and you would reach MDH fairly
close to the runway. However, if the wind was strong or unpredictable it
could muck up the descent and you could easily find yourself at MDH with =
mile of so to go - hence the need to stay at that height until the MAPt. =
could play safe and reduce the descent rate to stop this happening but =
could then be faced with the opposite problem in that you might still be
coming down to MDH as the airfield appeared in the murk below you - with =
too high to effect a landing.

So the MDH rule still applies to non precision approaches even though we
have the luxury of DME to show us exactly how close we are to an =
This makes such approaches much more safe and accurate and you would =
the DH rule could be applied to them but there is one more factor for
keeping the MDH. Autopilots can track and ILS and they can be programmed =
flag the DH and alert the pilots to land or go around. This still can't =
done with a non precision approach. All you can do with a NPA is use the
autopilot to fly the correct inbound heading but it can't be given any
descent guidance. Therefore you just dial up the MDH and let it take you
down to that height.

I think I'd better add that to the page...


-----Original Message-----
From: jhb_airlines-bounce@xxxxxxxxxxxxx
[mailto:jhb_airlines-bounce@xxxxxxxxxxxxx] On Behalf Of gwinsk@xxxxxxx
Sent: 23 November 2004 14:08
To: jhb_airlines@xxxxxxxxxxxxx
Subject: [jhb_airlines] Re: Article=20

Having read and re read the "Article", and learned a lot from it, may I =
a few=20

Very important. What's the aircraft featured in the first photograph? =
thrown by the=20
presence of what look like sidesticks, on to of a central control =

What is the Autopilot MCP? I know the answer is going to be =
obvious? Is=20
it Master Control Panel?

Not in the article but arising from it. I find that some of the FS jets =
reluctant to slow,=20
entailing a need to start the decelleration way ahead of the 10,000' =
or any other=20
speed control point. If the first stage of flaps can't be deployed until
down to 220kias,=20
then that just leaves the spoilers. On the IFSDG A320, the braking =
effect of
the spoilers=20
is minimal. I've looked through Aircraft.cfg and used AirEdit on the =
file, but can't find=20
a way of increasing the spoilers' drag. Any ideas?

I was sure I'd preserved your words of wisdom on the subject but can't, =
find them.=20
The info I'm seeking is on Decision Height and MAP differences and

Something else is sure to hit me, so I may be back. Meanwhile, off to =
Werner Schott's Checklists can be chopped down and stuck onto file card. =

disappearing under paper and can't find the thrust lever!

Gerry Winskill

Other related posts: