[jhb_airlines] Re: Article

  • From: gwinsk@xxxxxxx
  • To: jhb_airlines@xxxxxxxxxxxxx
  • Date: Wed, 24 Nov 2004 22:01:06 -0000

Perhaps a clue lies in a recent accident report. A flight doing a missed 
approach ran into 
high ground. It was found that both altimeters were on QFE "Not having been 
reset after 
the overshoot". Apparently one is supposed to be set on QNH, if the other has 
been set 
to QFE, which more or less confirms they have a decision height QFE on one 
instrument?

Gerry Winskill
gwinsk@xxxxxxx



On 24 Nov 2004 at 19:33, Bones wrote:

> Astute question. I was going to write back and say "yes" but then I
> had = to pause and think for a bit.=20
> 
> You see, the UK airlines almost all used QFE up until a few years ago
> = and 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. = That would mean a DH of 452ft QNH.
> 
> I had two thoughts here. One was that pilots add their DH to the =
> airfield 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 = web search and found evidence at
> http://av-info.faa.gov/terps/Policies1/TIL00005Aatt.PDF that DH is not
> = used with the Rad Alt unless the pilot knows the height of the
> terrain at the = DH location and adjusts for the difference between
> threshold elevation.
> 
> I conclude, for the moment, that DH is indeed set on the altimeter and
> = that the crews must have a QNH as well as QFE value on their plates
> to set. = I'll 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 = and therefore aren't quite as good as getting
> Aerad or Jepp plates..
> 
> 
> bones
> 
> 
> -----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, =
> We're only=20 given QNH by FPI controllers........
> 
> Gerry Winskill
> gwinsk@xxxxxxx
> 
> 
> 
> 
> 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.
> >=20
> > MCP
> > 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, =
> ALT,=20
> > Speed, VS, =3D APR, BC buttons etc or as the Go Flight MCP unit. As
> > =
> most=20
> > 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.
> >=20
> > 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 =
> becomes=20
> > 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.
> >=20
> > 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.
> >=20
> > 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 =
> would=20
> > take 20 min to =3D descend to sea level and at 3nm per minute this =
> would=20
> > 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.
> >=20
> > 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 =
> of=20
> > 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
> > - =
> I=20
> > 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
> > =
> the=20
> > aircraft back under the autothrottle envelope.
> >=20
> > Admittedly this will entail a longer descent profile and so you
> > should =3D plan for this. Alternatively use FSNav to compute the top
> > of =
> descent
> > - but =3D make sure you change the ROD in the aircraft's profile=20
> > first.=3D20
> >=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 =
> 
> > =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 =
> for=20
> > first stage setting =3D and there are probably others too.
> >=20
> > 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..
> >=20
> > 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..
> >=20
> > 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.
> >=20
> > 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.
> > =
> =3D=20
> > 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.
> >=20
> > 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
> > =
> it=20
> > 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
> > =
> are=20
> > much the same - 250ft for an =3D ILS, 400ft for a Loc/DME and 600ft
> > =
> for=20
> > most others.
> >=20
> > 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 =
> so=20
> > bear with =3D me whilst I explain again.
> >=20
> > 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 =
> Precision=20
> > Approaches (VOR, NDB and Localiser only) use a different point
> > called=20 the Minimum Descent Point. There are subtle differences.
> >=20
> > 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.
> >=20
> > 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 =
> gets=20
> > 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 =
> MAP)=20
> > is only necessary and shown =3D for non precision approaches.
> >=20
> > 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 =
> distance=20
> > 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
> >=20
> > The approach procedure was based on flying over the beacon at a set
> > =
> =3D
> > 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 =
> like=20
> > 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 =
> play=20
> > 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.
> >=20
> > 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 =
> =3D=20
> > 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.
> >=20
> > I think I'd better add that to the page...
> >=20
> > bones
> >=20
> > -----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
> >=20
> >=20
> > John,
> > Having read and re read the "Article", and learned a lot from it,
> > may I =3D ask a few=3D20 questions?
> >=20
> > 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?
> >=20
> > What is the Autopilot MCP? I know the answer is going to be =3D
> > embarrasingly obvious? Is=3D20 it Master Control Panel?
> >=20
> > 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 =
> speed=20
> > 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 =
> looked=20
> > 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?
> >=20
> > 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
> > =
> Height=20
> > and MAP differences and determination.=3D20
> >=20
> > 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 =
> and=20
> > stuck onto file card. =3D I'm
> >=20
> > disappearing under paper and can't find the thrust lever!
> >=20
> > Gerry Winskill
> > gwinsk@xxxxxxx
> >=20
> >=20
> >=20
> >=20
> 
> 
> 
> 



Other related posts: