Numerical
analysis and flight check
The numerical performance analysis
of the glidepath and the flight check are closely related in that respect
that the pathes for calculation and analysis and flight check should coincide.
DDM-errors in turn do depend inherently on the definition of the correct
(wanted) glidepath which depend in the classical approach on the ground
surface parameters, the mentioned forward and sideward slopes.
Modern tridimensional numerical methods
allow to analyse and to optimize the geometry of an ILS glidepath. By this,
non-ideal ground surfaces can be accepted to a certain extent. However,
an optimization is not possible in the entire coverage volume which is
in particular applicable for the glidepath performance in the azimuthal
angular coverage region.
The governing specifications is the
ICAO Annex 10. In some respect, Annex 10 is not complete and not adapted
to the actual requirements. E.g., this is for the azimuthal coverage of
the ILS-glidepath where a coverage of ±8° is specified. However,
it is not uniquely defined which specifications shall be applied in this
angular range. This is especially for the DDM. The DDM-tolerance specifications
on the glidepath (e.g. ±30µA, ±20µA CATII/III)
cannot be met in the entire angular depending on the antenna position,
the ground behaviour and the type of the glidepath-system.
A treatment af the intercept and
landing procedures today (Fig. 5) shows that the glidepath information
needed is related to the width of the associated Localizer. Shorter runways
have wider Loc-widths up to ±3°. In any case the aircraft is
intercepting first the Localizer, after that the glidepath
ususally from below (Fig. 5). The autopilot of the intercepting aircraft
shall not be engaged above 175µA acc. Annex 10. By this, it is reasonable
that the quantitative glidepath information is verified in an angular range
which is dependant on the related Localizer. E.g. the DDM-tolerance on
the glidepath is applied in an angular of the ±Loc-width and the
double tolerances are applied to double ±Loc-width angular range.
This is in case of a runway length of 4000m (+300m Loc distance) an angular
range of ±1.4° and ±2.8°. In case of very short runways
the corresponding maximum angles are ±3° and ±6°.
This principle has been applied to several ILS-optimizations for difficult
sites and grading treatment (see Fig. 3 and 4). However in turn, these
procedures should be applied also by the flight check procedures.
The most basic parameters of the
ILS-glidepath are the
-
glidepath angle Q0
-
crossing height HCR
-
DDM .
Both first parameters are the decisive
system installation figures. Annex 10 and the associated documents (DOC
8071) are describing the procedures to measure or to determine these. However,
in the numerical case an alternative method has been developed which is
the most adequate method. Fig. 6 shows schematically the method. The common
locus-curve of the DDM=0 is determined at N points between the points A
and B. The slope of the interpolated straight line results in the glidepath
angle and the extrapolated line determines the crossing height HCR
above threshold. A numerical cross-check is made during each numerical
analysis which determines the DDM=0 crossing height above threshold via
a vertical trace (Fig. 7). Both methods cannot be performed rigorously
by classical flight-check procedures. The first one (common locus DDM=0)
can be approximated in the flight check measurement by using the known
actual flight path data and applying approximately the displacement sensitivity.
However, a number of error sources are involved in this process.
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