My understanding of an ILS is that the only significant (powerful) false gideslopes occur above the normal beam at multiples of the glideslope angle (theta) e.g. 2 x theta, 3 x theta, which are 6 and 9 deg for a standard 3 deg slope.
Dear
alf5071h
Respectfully, I hope you are not confusing the difference between a false G/S and an erroneous G/S. Erroneous G/S can happen on
any ILS installation at
any time and are incredibly dangerous because the a/c equipment does not detect them as being erroneous.
The following rant is for the benefit of any pilot still out there who doesn’t understand why we carry the ILS Outer Marker Check (the topic of this thread):
All professional pilots should read and understand investigation reports such as why Air New Zealand NZ 60 on 30 July 2000 nearly crashed into the sea some 5 nm short of the runway, so that we may avoid been killed by a now “well known and understood” limitation of the standard ILS installation (see Shore Guy’s link above).
The problem with an erroneous G/S been radiated by an ILS installation which has been inadvertently being left in the maintenance/test mode (and is NOT DETECTABLE by the a/c instruments to warn the flight crew) identified by the report, HAS NOT BEEN FIXED.
The only way we pilots can verify a valid ILS glideslope before we trust our life and the lives of our pax using that G/S down to decision altitude/height (Minima), is to physically perform a check that the on-slope indication of the G/S intercepts a defined check altitude at a defined distance from touchdown.
The height check at the outer marker is shown on each ILS Jepp (approach) plate. At some installations the outer markers have been replaced by a defined DME distance instead. The procedure for the crew to carry out the check is defined in your company ops manual.
If the height does not check, the approach must be immediately discontinued.
The
height check is made by reading the
altitude on the barometric pressure altimeter. The
altitude you read at the outer marker may be slightly different to the true
height due to actual atmospheric differences to the standard (namely temperature).
As a guide to acceptable discrepancies for the check, in hot weather your indicated
altitude may be up to about 100 feet
lower than the
height shown on the approach plate at the marker. In cold weather the reverse applies. This is normal and to be expected.
Lastly to address the topic of cold weather corrections that may be needed to be added to altimeter readings at Minima due to the extremes of airfield temperature from standard…
This has nothing to do with the G/S validity check! Good airmanship dictates that any required changes to minima must be calculated and agreed upon in the cruise when destination airfield conditions are received, then briefed prior to the approach.
Corrections to minima should only be made in accordance with your company ops manual. However, for guidance, most companies will only require you to make a correction for very low destination airfield temperatures. In very low temperatures the
height of your a/c above touchdown at Minima will be significantly lower (>10%) than your indicated barometric
altitude and obviously this would be dangerous if you did not add a correction.
As a final note of interest, if your Minima is read from your Radar Altitude indicator (ILS CAT II/III), NO corrections are made or are necessary for any variations in temperature.
Even in this case the outer marker check height is still made from reference to the barometric altimeter altitude!