# THRESHOLD CROSSING HEIGHT -- A FRESH PERSPECTIVE.

# 

## Martyn Wills

Flight Precision Ltd

Teesside International Airport

Darlington

DL2 1NJ

England

# [ABSTRACT]{.underline}

It is evident, both from recent experiences, and from papers previously
presented at IFIS and other forums, that a degree of confusion exists in
both the understanding of threshold crossing height and how it is
measured.

To appreciate the applicability of threshold crossing height, both from
an operational perspective, and in terms of its measurement by flight
inspection, it is necessary to understand both the background of the
ICAO requirements, and how the ILS is used operationally.

Once these questions have been answered, it is essential that the method
of flight inspection measurement is commensurate with the parameter
requirements, both in terms of accuracy and method of measurement.

This paper seeks to understand the concepts surrounding the measurement
of threshold crossing height, and question the applicability of some of
those concepts.

# 

# [BACKGROUND]{.underline}

ICAO specifies criteria for ILS reference datum height and threshold
crossing height and attaches great importance to them. This paper
investigates the background to the ICAO requirements, looks at how we
measure the parameters, and questions the relevance of the application
of ILS reference datum height.

# [ICAO REQUIREMENTS]{.underline}

ILS Reference Datum Height (RDH), what is it? From the definitions
stated in ICAO Annex 10^1^: "A point at a specified height located above
the intersection of the runway centre line and the threshold and through
which the downward extended straight portion of the ILS glide path
passes." This seems quite straightforward, so why do we need it? ICAO
annex 10^1^ goes on to state in 3.1.5.1.3 "The downward extended
straight portion of the ILS glide path shall pass through the ILS
reference datum at a height ensuring safe guidance over obstructions and
also safe and efficient use of the runway served." This means that the
aircraft should be high enough to safely clear any obstructions on the
ground, but low enough to make the best use of the runway length
available. These two objectives are mutually exclusive. Safe obstacle
clearance would predicate a high threshold crossing height to maximise
terrain clearance, but this would reduce the runway distance available
for landing.

Annex 10^1^ goes on to state "The height of the ILS reference datum for
Facility Performance Categories II and III --- ILS shall be 15 m (50
ft). A tolerance of plus 3 m (10 ft) is permitted. In arriving at the
above height values for the ILS reference datum, a maximum vertical
distance of 5.8 m (19ft) between the path of the aircraft glide path
antenna and the path of the lowest part of the wheels at the threshold
was assumed." Annex 11^2^ appendix 5; table 2 further states that the
data accuracy for Threshold Crossing Height for precision approaches
shall be 0.5m or 1ft with an integrity classification of 1x10^-8^
(critical)

Instrument Approach Procedure designers use the ILS Reference Datum
Height to determine the obstacle clearance surfaces of the approach in
accordance with ICAO DOC8168^4^(Pans-Ops). This is why the reference
datum height is quoted as 15m with only a positive tolerance. If the
instrument approach procedure is constructed using a reference datum
height of 15m, then any increase in RDH serves to increase the safety
margin of the procedure. The problem with increasing the RDH is that for
a level runway with a 3° glidepath, each 1m increase in RDH pushes the
touchdown point 20m further down the runway. It is therefore desirable
to have the RDH as close to 15m as practicable.

# [RDH and TCH]{.underline}

ICAO Annex 10^1^ talks of ILS Reference Datum Height, or RDH. Threshold
Crossing Height is referred to as Achieved Reference Datum Height
(ARDH). It is often assumed that the two are the same. Indeed, in an
ideal world, with perfectly flat terrain, they would, to all intents and
purposes be the same. To understand TCH, it is best to look at how the
glidepath is used by the automatic flight control systems during
autoland. The data used here is for the Smiths SEP6 autopilot, the first
system certified for Cat III autoland.

During descent from glidepath intercept, the aircraft vertical guidance
channel follows the glidepath beam. At 133' on the radio altimeter
(1600' prior to threshold), glidepath guidance is disconnected, and the
aircraft enters an 'attitude-hold' phase, having memorised and averaged
the previous 10 seconds of aircraft attitude data. This continues until
70' on the radio altimeter, when the radio altimeter takes over the
vertical guidance, flaring out the aircraft for landing. For a typical
aircraft approach speed of 175kts, the part of the glidepath that
determines the TCH is therefore the portion between 4500' and 1600' from
threshold. This may be different to the extended straight-line portion
between points 'A' & 'B' used to calculate RDH.

Attachment C to volume 1 of Annex 10^1^recognises this difference, and
specifies that the measurement of ARDH be calculated over the segment of
6000' to 1000' prior to threshold. Although recognising the operational
significance, annex 10 makes no specifications for ARDH; however, Annex
11^2^ specifically states the data quality requirement for TCH.

Where there is a significant cross-slope and forward-slope between the
glidepath and the runway, the glidepath structure may show a pronounced
early flare characteristic. In this instance, there may be marked
differences between RDH and ARDH/TCH. In most cases, TCH is higher than
RDH due to the curvature of the glidepath, but this is not always so. If
the glidepath exhibits marked 'negative flare', the ARDH may be
significantly lower than the RDH. The older version of DOC8071^3^
specifically mentioned the undesirability of negative flare; the current
edition makes no reference to it. Annex 10^1^ states:--- "In regions of
the approach where ILS glide path curvature is significant, bend
amplitudes are calculated from the mean curved path, and not the
downward extended straight line." It is assumed from the basic physics
of the glidepath, that the curved portion will be upwards. Any tendency
to downward curvature will have a detrimental effect on ARDH/TCH, but
may have little effect on RDH.

# [RELEVANCE OF ILS REFERENCE DATUM HEIGHT]{.underline}

When the concept of autoland using ILS was introduced in the early
1960s, glidepath siting and the determination of RDH by flight
inspection were both carried out by simple geometry of glidepath back
set distance, threshold elevation relative to the glidepath, and mean
glidepath angle as calculated using optical tracking techniques. To the
best of my knowledge, there has not been any grave safety concerns
raised by the use of this simple method during the past 40 years
experience of autoland.

Considering the potential errors in measurement by all of the variables
concerned in calculation of RDH, it may lead to questioning the accuracy
requirements for RDH measurement. Indeed, do we need to know the value
for RDH?

The purpose of the ILS RDH is twofold:

1.  The basis for the calculation of the obstacle assessment surfaces
    for the instrument approach procedure design. In this instance, the
    ILS RDH is assumed to be 15m. (Pans-Ops^4^ 21.1.3). ICAO does allow
    for a higher RDH to be promulgated for reasons of obstacle clearance
    in attachment C to Vol 1 of Annex 10^1^.

> The obstacle assessment surfaces are calculated on a RDH of 15m. This
> allows a target level of safety of 1x10^-7^ to be achieved. If the RDH
> is higher than 15m, then safety is increased.

However, the glidepath might radiate a lower than promulgated angle and
yet remain within the operating tolerances. In this instance, the RDH
could be lower than 15m. This would invalidate the basic premise of the
obstacle surface assessment and the instrument approach procedure
design.

2.  To provide a safe threshold crossing height for aircraft whilst
    making maximum use of the available runway. Annex 10^1^ assumes a
    maximum of 5.8m between the path of the aircraft glide path antenna
    and the path of the lowest part of the wheels at the threshold.
    Additionally, a maximum vertical glidepath displacement due to
    perturbations of 1.2m is assumed. Thus with a 15m RDH, there should
    be a clearance of 8m between the aircraft wheels and the runway
    surface at threshold.

> The important factor here is ARDH or TCH. The ARDH should be high
> enough to preserve obstacle clearance but the final path angle over
> the range 6000' -- 1000' is also important in determining the
> touchdown point.
>
> For cat I operations, where the decision height is not less than 200',
> the aircraft is 3,800' or 1,160m from touchdown at decision height.
> The importance of ARDH in this instance is quite low provided that the
> aircraft is in a stable attitude at the decision height whereby the
> pilot can take control and land using visual references.
>
> For Cat II operations, the decision height may be as low as 50'. In
> this instance, ARDH is very important to ensure that safe obstacle
> clearance is maintained and the aircraft is well positioned in
> relation to the touchdown point as the pilot has little time for large
> corrections to the aircraft path after decision height.
>
> The Cat III case was examined earlier. Here, the ARDH is very
> important to ensure safe obstacle clearance; also, the final path
> angle, and the runway slope from threshold will influence location of
> the touchdown point.

# [MEASUREMENT OF REFERENCE DATUM HEIGHT]{.underline}

The accuracy of measurement of RDH depends on the method of measurement
used, and to a certain extent, the interpretation of what RDH actually
is.

Before the advent of computer analysis of flight inspection data, the
calculation of RDH was simply a geometric calculation based on the mean
measured glidepath angle, the back set distance of the glidepath from
threshold, and the difference in elevation between glidepath and
threshold. This is the same process outlined in attachment C to part 1
of annex 10^1^ to determine the siting of the glidepath equipment.

When measuring glidepath angle with a theodolite or other optical
tracking system, the derived angle is dependant on the position chosen
for the siting of the measurement system. Most of these systems are
sited based on the premise that the glidepath signal originates from the
base of the glidepath mast. The physical height of the tracking system
dictates that the measurement point is some distance ahead of the
glidepath such that the measurement device is located in the plane of
the nominal glidepath. This is further complicated by the placement of
the measurement system towards the runway, but away from the runway
centreline. Automatic flight inspection systems define an 'aiming point'
from which all angular calculations are made. This is normally a
theoretical point on the runway centreline abeam the glidepath
equipment, at the same elevation as the base of the glidepath mast, from
which the glidepath signal is assumed to originate.^6^

It can be seen from the (exaggerated) diagram above that several
interpretations of RDH are possible. Firstly there is the line of
glidepath 0ddm, the dashed line on the diagram. There is the arithmetic
mean angle as measured by the theodolite system, the double line on the
diagram. The Best Fit Straight Line (BFSL) between points A and B, and
the TCH as measured between 6000' and 1000', either as an arithmetic
mean, or by BFSL.

This gives two possible values for RDH, and two possible values for TCH,
depending on the method used for calculation.

The siting of the tracking device as mentioned earlier, or definition of
the 'aiming point' in automatic flight inspection systems, further
complicates this situation. It is from these points that all angular
calculations are made; therefore the choice of these points is
fundamental to the calculation of RDH/ARDH.

The siting of optical tracking devices is at best an engineering
judgement based on assumptions of the origin of the glidepath signal and
estimations of the local topography. The procedures for determining this
point vary from state to state.

The calculation of an aiming point for automatic flight inspection
systems still assumes the origin of the glidepath signal, but does bring
some degree of uniformity in measurement. There have been attempts to
refine this process further in recent years by calculating the BFSL
between points A and B^6^, and using this data to redefine the aiming
point, allowing a recalculation of angle, structure and RDH/ARDH.
Although there is some merit in this process, the experience of Flight
Precision Ltd is that the results obtained do not show a great deal of
repeatability for more unusual glidepath siting conditions compared to
'traditional' methods, and the spread of values is sometimes quite
large. Where there are significant distortions to the overall glidepath
structure, this method can produce excessive corrections to the
elevation of the glidepath aiming point as calculated from the BFSL. In
these situations, differences of up to 6m in calculated RDH have been
noted. The question also remains as to which is the correct value of
RDH?

Although this refined process allows for a redefinition of the aiming
point for RDH, it does not allow a redefinition of the aiming point for
calculation of ARDH/TCH. Thus the inherent errors in ARDH/TCH still
remain. If the glidepath curvature is upwards, then ARDH/TCH should be
higher than RDH, and safe obstacle clearance will be preserved. If the
glidepath exhibits a downward curvature prior to threshold, then TCH and
obstacle clearance may be compromised.

A more fundamental matter to the question of measurement of RDH is the
definition of threshold itself. National Authorities, individual
airports, and flight inspection organisations all have different
interpretations on the definition of threshold. The DOC8168^4^
definition is 'The beginning of that portion of the runway useable for
landing.' The application of that definition means different things to
different people. To illustrate the problem, FPL asked the UK CAA to
make a statement on the definition of threshold, and three different
departments had three different views, the answer is not so obvious.
Changing the definition of threshold from the leading edge of the 'piano
keys' to the threshold lighting bar (typically 6m) changes the RDH
calculation by 0.3m for a 3° glidepath.

A further observation is the relationship between the glidepath angle,
within the operating tolerances, and RDH. The graph below illustrates
the effect of changing the glidepath angle on the RDH. This assumes a 3°
glidepath with a RDH of 15.0m when operating at the promulgated angle.

It can be seen that the change in RDH with glidepath angle is
significant, even though the glidepath remains within the operating
tolerances. Calculated RDH may change by ±1.1m from the nominal with the
glidepath remaining within Cat I/II angular operating tolerances, and
±0.6m for Cat III. Can one assume that if the RDH is not less than 15m
at the promulgated glidepath angle, the obstacle surface assessment
safeguards the operation to the limits of tolerance of the glidepath
angle?

# [FLIGHT INSPECTION ISSUES]{.underline}

![](media/image1.wmf){width="3.0in"
height="1.7680555555555555in"}Problems currently arise in the
measurement of RDH/ARDH because the technology is available to the
flight inspection organisations to calculate these parameters by more
advanced methods than were previously possible, and come up with answers
that may be significantly different from traditional methods. ILS
installation engineers are also able to use advanced computer modelling
techniques to optimise glidepath siting by three dimensional terrain
modelling rather than the more basic methods outlined in annex 10. This
gives rise to incompatibilities between the flight inspection
measurement methodology and the ILS siting criteria.^7^

It is not reasonable to site an ILS for a planned RDH by one method, and
then validate the RDH value by flight inspection using a totally
different method. The two methods are incompatible.

It is also irrational to henceforth change the flight inspection
measurement methodology and find that Cat III ILS systems that have been
operating satisfactorily for more than 30 years no longer meet ICAO
requirements purely because the flight inspection measurement philosophy
has changed.

There is a requirement for greater understanding between the ILS
installation engineers and the flight inspection organisations to ensure
that the methods of measurement are appropriate to validate the premise
on which the planned ILS performance is based.

# [CONCLUSIONS]{.underline}

It is clear that RDH is critical to the obstacle surface assessment for
ILS Instrument Approach Procedure design, but from Pans-Ops, the actual
value is not critical provided it is not less than 15m. The actual value
only becomes critical if a higher value of RDH has been chosen to
facilitate obstacle clearance and this must be validated.

If safeguards are built in to Pans-Ops obstacle surface assessment
criteria to allow for the glidepath angle tolerance, then any assessment
of RDH should be corrected to the promulgated glidepath angle rather
than that existing at the time of the flight inspection.

RDH is published in the facility data for an ILS in national and
international publications. This is largely irrelevant if the RDH
exceeds the 15m that the obstacle surface assessment has been based on.
Aircraft operations are much more concerned with ARDH, which is rarely
published. It may be better to publish all ILS RDH as 15m, only quoting
the measured RDH if it falls outside of ICAO criteria.

The current diversity of measurement methods and the subsequent results
is undesirable. There is a need to agree a common methodology of
measurement that is consistent regardless of the flight inspection
service provider or type of flight inspection system used. It is
irrational to henceforth change the flight inspection measurement
methodology and find that Cat III ILS systems that have been operating
satisfactorily for more than 30 years no longer meet ICAO requirements
purely because the flight inspection measurement philosophy has changed.
This factor must be considered before changes are implemented.

It is the TCH parameter that so much importance is attached to in Annex
11^2^, but experience would indicate that the accuracy required is not
achievable with the diverse methods of measurement used for flight
inspection, and the glidepath angle tolerances are much wider than the
desired tolerance on TCH.

ARDH/TCH is an important parameter for the operation of the aircraft and
the behaviour of the aircraft during autoland. The path angle during
this final segment also has a bearing on the safe operation and the
location of the touchdown point. The difficulty here is the accuracy and
applicability of the measurement by flight inspection. The obvious
method of validation is to equip the flight inspection aircraft with a
certified category II autopilot capable of coupled approaches to an MDH
of 50', and assess the threshold crossing height directly. This is
probably impractical and the cost prohibitive for most flight inspection
companies, therefore, thought must be given to standardisation of
measurement that results in consistent and accurate determination of
TCH.

The philosophy used for ILS siting criteria must be commensurate with
the flight inspection methodology used for validation of parameters such
as RDH and vice-versa. If the flight inspection community is to adopt
advanced techniques of determining ILS parameters such as those outlined
in FAA Order 8240.47C^5^, then the ILS siting criteria and modelling
techniques must also change to take this into account.

This paper raises several issues, and leaves many questions unanswered.
The situation whereby the measurement of a parameter specified as
critical by ICAO is open to so much interpretation and differences due
to measurement techniques cannot continue. Rather than prescriptive
regulation of flight inspection, it may be better to fundamentally
review the ICAO requirements regarding the applicability of RDH and
ARDH/TCH, together with the associated influencing factors, before
deciding on the optimum method for validation. What is needed is a fresh
perspective.

# [REFERENCES]{.underline}

1.  ICAO Annex 10

Aeronautical Telecommunications Vol I

2.  ICAO Annex 11

> Air Traffic services

3.  ICAO DOC 8071

> Manual on testing of radio navigation aids

4.  ICAO DOC 8168

> Pans-Ops

5.  FAA Order 8240.47C

> Determination of RDH

6.  11^th^ IFIS Proceedings

Methods of computing glidepath characteristics in modern flight
inspection systems - Buchheim

7.  11^th^ IFIS Proceedings

Problems & solutions for ILS Cat III airborne & ground measurements --
European & US views and perspectives. Greving/Spohnheimer