[jhb_airlines] Re: Article

  • From: "Bones" <bones@xxxxxxx>
  • To: <jhb_airlines@xxxxxxxxxxxxx>
  • Date: Wed, 24 Nov 2004 19:33:56 -0000

Astute question. I was going to write back and say "yes" but then I had =
pause and think for a bit.=20

You see, the UK airlines almost all used QFE up until a few years ago =
then most migrated to the US policy of remaining on QNH.  If landing QNH
then ATC don't pass the QFE value - and so Decision Height can't be QFE
related. Or so the logic told me, but it also sounded wrong.

Looking up the DH of 27L at Heathrow gives a value of 250ft and this =
must be
a QFE value (height above airfield) as Heathrow is about 202ft AMSL. =
would mean a DH of 452ft QNH.

I had two thoughts here. One was that pilots add their DH to the =
elevation and the other was that they set DH on the Rad Alt. In fact it
seemed logical that the latter was the sensible route but then I worried
about sloping ground affecting Rad Alt readings. I had to resort to a =
search and found evidence at
http://av-info.faa.gov/terps/Policies1/TIL00005Aatt.PDF that DH is not =
with the Rad Alt unless the pilot knows the height of the terrain at the =
location and adjusts for the difference between threshold elevation.

I conclude, for the moment, that DH is indeed set on the altimeter and =
the crews must have a QNH as well as QFE value on their plates to set. =
have to look further as a read through the UK AIP hasn't provided any
clues.. Just accept that the CAA plates avoid any reference to DH or MDH =
therefore aren't quite as good as getting Aerad or Jepp plates..


-----Original Message-----
From: jhb_airlines-bounce@xxxxxxxxxxxxx
[mailto:jhb_airlines-bounce@xxxxxxxxxxxxx] On Behalf Of gwinsk@xxxxxxx
Sent: 24 November 2004 17:09
To: jhb_airlines@xxxxxxxxxxxxx
Subject: [jhb_airlines] Re: Article=20

Presumably Decision Height relies on QFE? If memory serves me right, =
given QNH by FPI controllers........

Gerry Winskill

On 24 Nov 2004 at 5:10, Bones wrote:

> Panel
> 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 =3D of switches on the default FS aircraft covering the HDG, =
> Speed, VS, =3D APR, BC buttons etc or as the Go Flight MCP unit. As =
> add on aircraft =3D 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=20
> confusion.
> Speed control
> Speed behaviour in FS is pretty accurate and you hit the same problem
> =3D found in the real world. In fact slowing down in the descent =
> a problem even down to GA levels once you get a fairly streamlined=20
> aircraft. A Cherokee or C172 does not have any problem but move up to=20
> the PA24 =3D Comanche or PA31 Navajo and you get a shock.
> The basic dilemma is that sleek aircraft cannot descend and slow down
> at =3D the same time. Even with much reduced throttle many aircraft=20
> remain close to cruise speed when descending and so you are in the=20
> trap of foregoing one =3D for the other - it is either rate of descent =

> or airspeed that you opt for. =3D You can't do both.
> The trick is to ignore the book figures for descent rate for the =3D
> aircraft and use something less. In the PA31 we used a 500 fpm descent =

> rate at =3D 180kts as this is easy to calculate. If up at FL100 it =
> take 20 min to =3D descend to sea level and at 3nm per minute this =
> mean starting down some =3D 60nm from destination. Also, at 500fpm, we =

> had a bit of flexibility as we =3D could still slow down a bit or we=20
> 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=20
> see what ROD they use so the above is just a guideline. At the=20
> shallower =3D descent rate the autothrottle would probably have a bit =
> power on and so you'd have a small degree of control over speed=20
> settings. An example is one of =3D the default aircraft (747 or 737 - =
> can't recall) that won't peg at 250 kts during normal autopilot=20
> descent but stays up nearer 270/280kts. Reducing =3D ROD will bring =
> aircraft back under the autothrottle envelope.
> Admittedly this will entail a longer descent profile and so you should
> =3D plan for this. Alternatively use FSNav to compute the top of =
> - but =3D make sure you change the ROD in the aircraft's profile=20
> first.=3D20
> 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 =

> =3D descent point. Flaps are generally no help but again this varies=20
> with aircraft =3D type - I think the 747 has a high figure of 270kts =
> first stage setting =3D and there are probably others too.
> Spoilers
> 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 =3D=20
> whereas 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=20
> have been removed as they are now considered company minima. The=20
> Jepp/Aerad charts still give the data (airlines then factor this for=20
> local use) but =3D you don't have to resort to searching these out. I=20
> will explain..
> A few years ago the CAA was criticised for not stipulating minima =3D
> because this allowed non company aircraft (business or private
> aircraft) to =3D approach in far worse conditions than the airlines or =

> charter aircraft (which =3D were legally bound to publish minima for=20
> their pilots). A lot of other =3D countries had already produced State =

> Operating Minima and the CAA eventually did =3D the same.
> 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. =
> Scroll down to Section 2.22 Flight Procedures and look for sub section =

> 6 - Aerodrome Operating Minima for Non Public Transport Flights. This=20
> 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
> =3D and 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 =3D radar approach (SRA) has a much higher limit - 600ft - because =
> is deemed a =3D non precision approach, as are NDB or VOR letdowns. If =

> you look at Heathrow =3D (and almost all other airports) the limits =
> much the same - 250ft for an =3D ILS, 400ft for a Loc/DME and 600ft =
> most others.
> The Missed Approach Point is shown on the approach plate but not for
> an =3D ILS approach as it is redundant. This goes back to definitions =
> bear with =3D me whilst I explain again.
> An ILS is defined as a Precision Approach. Because of this the =3D
> terminology is slightly different in that the point at which the=20
> approach is =3D terminated is called the Decision Height. Non =
> Approaches (VOR, NDB and Localiser only) use a different point called=20
> the Minimum Descent Point. There are subtle differences.
> With the ILS the aircraft is allowed to descend to the DH (usually 250
> =3D ft) and, if the crew don't see the runway or approach lights they=20
> HAVE to execute a missed approach. In other words there isn't a MAPt=20
> 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
> =3D then it is allowed to level off and remain at the MDH until it =
> to the =3D MAPt - only then does it go around. So, a MAPt (I'm adding=20
> the little T because =3D the charts now show it as MAPt rather than =
> is only necessary and shown =3D for non precision approaches.
> If you are wondering about the oddity of the non precision approach
> then =3D let me explain. In the old days prior to DME a VOR or NDB=20
> approach was flown =3D on timings only - pilots didn't have an =
> information to show how =3D close they were to touchdown. This still=20
> applies to airfields without DME too =3D - it isn't a historical issue =

> (look up the NDB plate for Cranfield).=3D20
> The approach procedure was based on flying over the beacon at a set =
> height and flying outbound for several minutes - the exact timing=20
> depending on =3D your approach speed. You then turned inbound and=20
> descended to the MDH. If you =3D got it right this would almost be =
> 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=20
> you could easily find yourself at MDH with =3D a mile of so to go -=20
> hence the need to stay at that height until the MAPt. =3D You could =
> safe and reduce the descent rate to stop this happening but =3D you=20
> could then be faced with the opposite problem in that you might still=20
> be coming down to MDH as the airfield appeared in the murk below you - =

> with =3D you 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 =
> airfield. This makes such approaches much more safe and accurate and=20
> you would =3D think the DH rule could be applied to them but there is=20
> one more factor for keeping the MDH. Autopilots can track and ILS and=20
> they can be programmed =3D to flag the DH and alert the pilots to land =

> or go around. This still can't =3D be done with a non precision=20
> approach. All you can do with a NPA is use the autopilot to fly the=20
> correct inbound heading but it can't be given any descent guidance.=20
> Therefore you just dial up the MDH and let it take you down to that=20
> height.
> I think I'd better add that to the page...
> bones
> -----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:=20
> [jhb_airlines] Re: Article=3D20
> John,
> Having read and re read the "Article", and learned a lot from it, may
> I =3D ask a few=3D20 questions?
> Very important. What's the aircraft featured in the first photograph?
> =3D I'm thrown by the=3D20 presence of what look like sidesticks, on =
to of=20
> a central control =3D column?
> What is the Autopilot MCP? I know the answer is going to be =3D
> embarrasingly obvious? Is=3D20
> it Master Control Panel?
> Not in the article but arising from it. I find that some of the FS
> jets =3D are reluctant to slow,=3D20 entailing a need to start the=20
> decelleration way ahead of the 10,000' =3D point, or any other=3D20 =
> control point. If the first stage of flaps can't be deployed until=20
> down to 220kias,=3D20 then that just leaves the spoilers. On the IFSDG =

> A320, the braking =3D effect of the spoilers=3D20 is minimal. I've =
> through Aircraft.cfg and used AirEdit on the =3D .air file, but can't=20
> find=3D20 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, =3D now, find them.=3D20 The info I'm seeking is on Decision =
> and MAP differences and determination.=3D20
> Something else is sure to hit me, so I may be back. Meanwhile, off to
> =3D see whether=3D20 Werner Schott's Checklists can be chopped down =
> stuck onto file card. =3D I'm
> disappearing under paper and can't find the thrust lever!
> Gerry Winskill
> gwinsk@xxxxxxx

Other related posts: