[jhb_airlines] Re: Decision height

  • From: "Bones" <bones@xxxxxxx>
  • To: <jhb_airlines@xxxxxxxxxxxxx>
  • Date: Thu, 27 Oct 2005 19:52:09 +0100

Another can of worms has just been opened.. <vbg>

As Peter says it is not a specific figure as it will depend on airfield
equipment, aircraft type and pilot qualifications. To give just one example,
the DH for a CAT3 ILS will depend on the standby generators at an airport -
specifically their cut in time. You don't want to be down to 50ft above the
runway and have the ILS signal drop out because of a power failure so every
airport has to have standby generators and these must come in quickly. The
cut in time can determine the categorisation of the ILS. So do other factors
like approach lighting and runway lighting but I'm getting too deep now.

If you want to know how aerodrome minima works the basics can be found in
the AIP. Go to the AIS site and bring up the Aerodrome section but go to
Aerodromes General rather than Aerodromes Specific. Click on the page that
says AD1.1 Aerodromes/Heliports Availability (the CAA have got the link
wrong here). This will link to an 18 page PDF file.

Skip over the first five pages until you get to the section called Aerodrome
Operating Minima. The rest of the section covers the whole area of arrival
and departure minima and how they are worked out. It's good reading if a bit
deep and it will certainly give you an understanding about minima. For GA
pilots (I should really say non public transport flights) there are slightly
different rules and these are contained in CAP507 Aerodrome Operating Minima
For Private Pilots. It's a pity that this is not one of the free downloads
from the CAA publication library as it is also very good reading.

For individual airfields the UK have introduced what is known as State
Operating Minima. If you don't have a company minima or haven't worked out
(as Peter did) the minima for a specific approach then ATC should advise you
of the state minima for a specific approach. This information is published
in the Aerad and Jepp docs and I think it's also in Pooleys (it used to be)
- but it isn't in the UK AIP any more as far as I can see. It used to be
published in Section 2.22 for each airport but it isn't there any more. I'll
have to find out why - I guess the rules may have changed.

For a very basic guide for your FS flying then you can set an MDH of 450ft
for a LOC/DME, VOR or NDB approach and a DH of 250ft for an ILS. That's AAL
of course. <vbg>..


-----Original Message-----
From: jhb_airlines-bounce@xxxxxxxxxxxxx
[mailto:jhb_airlines-bounce@xxxxxxxxxxxxx] On Behalf Of Peter Dodds
Sent: 27 October 2005 16:06
To: jhb_airlines@xxxxxxxxxxxxx
Cc: pdodds@xxxxxxxxxxxx
Subject: [jhb_airlines] Re: Decision height

The Decision Height DH or Minimum Descent Height MDH isn't on the
charts because it is a calculated value for different conditions
and different airfields.
For a precision approach, the DH is defined
For a non-precision approach, the MDH is defined.
The definitions differ according to the pilot's qualifications.

For a precision approach, the minimum height to which a pilot
must descend before executing a missed approach is referred to as
the Decision Height.  (For a non-precision approach, it is
referred to as Minimum Descent Height).  The DH depends on a
number of factors.  Using Hawarden Runway 23 ILS Approach Plate
for example: -
a)      Obstacle clearance height (OCH) for a category A aircraft
with a 3% climb grade performance capability is 300 feet above
aerodrome level (QFE).
b)      Add the aerodrome threshold elevation (from the approach
plate) (17 feet) to obtain the OCH above sea level of 317 feet
c)      The minimum descent height for an ILS approach is 200
feet aal.  Check which of a) or b) is the greater.  In this case
it is a).
d)      Is an altimeter correction necessary? For a precision
approach, an altimeter correction of +50 feet is required.
e)      IMC rated pilots must add 200 feet to the minimum descent
height thus far calculated.  The minimum is therefore increased
to 550 feet aal.
f)      The absolute minimum descent height for an IMC rated
pilot on an ILS approach is 500 feet aal.  Check if d) is greater
than 500.
g)      In this case it is, so the Decision Height (DH) is 550
feet, and the Decision Altitude (DA) is 567 feet.

Bones will no doubt be able to add to this in respect of CAT I,
CAT II and CAT III ILS approaches.


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