[jhb_airlines] Re: Decision height

  • From: pdodds@xxxxxxxxxxxx (Peter Dodds)
  • To: jhb_airlines@xxxxxxxxxxxxx
  • Date: Thu, 27 Oct 2005 16:06 +0100 (BST)

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 
(QNH).
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.

Petr
 

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