AMOFSG/7-SN No. 23
12/8/08
AERODROME METEOROLOGICAL OBSERVATION AND FORECAST
STUDY GROUP (AMOFSG)
SEVENTH MEETING
Montréal, 9 to 12 September 2008
Agenda Item 5: Aerodrome observations
ALTIMETER SETTING REPORTS AND QUALITY ASSURANCE
(Presented by Bill Maynard)
SUMMARY
This paper reviews the Canadian experience with the identification of, and
response to, the occurrence of altimeter setting errors in METAR / SPECI.
These errors have been reduced from a maximum frequency of occurrence of
nearly 70 such errors a month to 15 to 20 in recent months. The most common
factors that contributed to the errors and the corrective actions that were taken
are reviewed. It is concluded that quality assurance measures are effective in
mitigating this situation.
1.
INTRODUCTION
1.1
The Aerodrome Meteorological Observing and Forecasting Study Group (AMOFSG)
was established by the International Civil Aviation Organization (ICAO) Air Navigation Commission
(ANC) in order to assist the Secretary from the meteorological section of the Air Navigation Bureau for
the purpose of re-validating existing practices in Annex 3 for observations and to identify improvements
in existing observation and forecasting standards and recommended practices. This Study Group is the
successor to the Aerodrome Meteorological Observing Systems Study Group (AMOSSG) and has carried
forward its work program with the addition of an explicitly wider scope to include forecasting topics.
1.2
The availability of an accurate aircraft altimeter setting is a safety critical issue in the
conduct of instrument procedures and is necessary for a pilot to conduct an instrument approach.
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An erroneous altimeter setting could result in a reduction in the necessary clearance over obstacles during
an approach and a consequential unacceptable increase in the risk of collision with an obstacle or
controlled flight into terrain (gross undershoot).
1.3
Errors in the reported value of the altimeter setting were previously thought to be
exceedingly rare. However, the introduction of additional quality control and monitoring processes to
METAR and SPECI production in the late 1990’s, coincident with the commercialization of the Canadian
Air Navigation System, revealed the existence of up to 70 errors per month in the altimeter setting. This
was immediately recognized as a safety hazard and actions were taken which reduced the frequency of
occurrence to 15 to 20 per month, most of which are corrected within the hour of occurrence, as
illustrated in Annex A. However there remain about 5 per month that are not corrected, most of which are
from smaller isolated aerodromes. However, these sites also tend to have non-precision approaches
increasing the potential risks associated with any altimeter setting error. Therefore continued efforts to
completely eliminate this hazard are ongoing.
2.
DISCUSSION
2.1
Risk estimation
2.1.1
The use of erroneous high altimeter setting reports may result in a pilot descending to
below minimum descent altitudes with a concurrent reduced clearance from obstacles and terrain. Most,
if not all, of the following factors would probably have to be present for this to result in a collision with
an object or controlled flight into terrain:
a) the occurrence of an altimeter setting error which is too high;
b) an approach conducted in weather conditions with visibility that is near or below
minimum or, in a worst case scenario, with zero visibility at the surface;
c) pilot unfamiliarity with the aerodrome which may be in mountainous terrain with
numerous obstacles in the vicinity;
d) the pilot does not recognize or correct for the fact that the aircraft is below glidepath
nor recognize that the decision height based upon the altimeter setting is too low. An
absence of any useable radar altimeter could be a factor;
e) the potential for additional complicating factors such as tailwinds on approach or
landing, windshear, crosswinds, icing conditions or turbulence;
f) low fuel or technical problems onboard the aircraft; and/or
g) very cold weather conditions.
2.1.2
The probability of an altimeter setting error occurring within a METAR or SPECI, prior
to any mitigating action, was roughly one per 3,000 observations. However, only half of these errors
were on the unsafe side. The greatest risks would be associated with cases when the altimeter setting was
erroneously high by about the same equivalent height as the decision height is above ground coincident
with zero effective visibility at the surface. Roughly 1 in 20 altimeter setting errors have been found to be
of that magnitude and sign. However, the incidence of true zero visibility is typically quite low
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AMOFSG/7-SN No. 23
(a representative occurrence, although widely variable, would be about 0.1 percent of the time leaving the
total exposure to the worst case situation – prior to mitigation - at about 1 in 60 million which is low but
clearly unacceptable given the potential severe consequences combined with the relative simplicity of
mitigation).
2.1.3
Most international airliners are equipped with radar altimeters and ground proximity
warning systems otherwise pilots, absent a visual reference, do not have any viable means to recognize
altimeter setting errors (except perhaps for gross errors). An ILS approach provides course guidance that
protects the aircraft down to the decision height, which may be erroneously too low in the event of an
error in the altimeter setting. However, unless the surface visibility is zero, the pilot will probably
recognize the situation before descending to the surface and correct the flight path. Furthermore, it is
notable that the existence of an effective “approach ban” regulation, assuming the availability of suitable
weather information to support it, would greatly limit exposure to this scenario.
2.1.4
The consequence of this risk scenario is most likely a missed approach with less than
designed clearance from obstacles or an unstabilized approach during short final with the possibility of a
hard landing. However, if there were additional factors related to pilot distraction and human factors, an
absence of warning systems, aircraft mechanical of systems failures or adverse weather then there is a risk
of collision with the ground or objects.
2.1.5
Accidents have been known to occur as a result of erroneous altimeter settings being used
on approach and they are clearly a relevant consideration in reducing the frequency of controlled flight
into terrain incidents. The following are excerpted from reports issued by the Canadian Transportation
Safety Board (TSB).
Over an 11-year period (1984-94) 106 persons lost their lives and 23 persons were
seriously injured in 70 accidents involving commercially operated aircraft that were
flown into terrain, under control, while the crew had no awareness of the impending
disaster. [TSB Communiqué A 96/12 refers]
The crew most likely conducted the instrument approach with reference to an
unintentionally mis-set altimeter of 30.17 in. Hg, and unknowingly flew the aircraft into
the water. The circumstances leading to the incorrect altimeter setting could not be
determined, nor was it determined why the crew did not detect the mis-set altimeter.
[TSB Report A95P004 refers]. [Note: -Although not attributed to an error in the METAR,
this event illustrates the potential sequence of events that could follow from it.]
2.1.6
Based upon consideration of all these factors it is concluded that the risk of accidents due
to altimeter setting errors is unacceptable and must be mitigated. Furthermore, as will be illustrated
below, a large measure of mitigation can be readily achieved in a short time frame and with low cost.
Moreover, these measure will likely benefit other aspects of aviation weather services.
2.1.7
It should be noted that altimeter setting errors that are too low give an increased decision
height and a resultant increased probability of a missed approach and potential unnecessary diversion to
another aerodrome. The associated risks, in this case, are primarily in the form of additional costs to the
operator and inconvenience to passengers.
AMOFSG/7-SN No. 23
2.2
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Detection Methods
2.2.1
It was immediately recognized upon discovery of these errors that methods must be
developed to ensure the timely detection of all suspicious altimeter setting reports in METAR/SPECI.
Some, or all, of the following methods can be implemented to ensure a maximum probability of the
detection of an anomalous altimeter setting report:
a) systematic and universal monitoring and archiving of altimeter setting reports;
b) gross error and code compliance checks (values that are not realistic are suppressed);
c) trend analysis to identify any altimeter setting that differs from a previous reported
value by an amount that is not realistic (this is not effective for the first one or two
observations of the day from aerodromes that do not have 24 hour per day
METAR/SPECI programs);
d) consistency with any associated mean sea level pressure reports;
e) any altimeter setting that would suggest an unrealistic pressure gradient in
comparison with surrounding stations can be flagged as suspicious;
f) a maximum degree of inconsistency with numerical weather forecast values of the
sea level pressure could be allowed before a report is flagged as suspicious;
g) station inspections routines can be modified to emphasize pressure equipment, assure
up to date conversion tables (where necessary) and ongoing compliance by staff with
operations procedures;
h) communications pathways can be established so that concerns from pilots and
meteorological personnel, regarding the altimeter setting, can be reviewed (pilots
manoeuvring on the surface may note a suspicious deviation between the altimeter
reading and the known surface elevation); and
i)
computer software can be developed which provides immediate alerts to the observer
of the most common errors or unusual trends.
2.2.2
In practice, these methods are primarily reactive therefore, with the exception of the final
method whereby software intervenes in the event of suspicious data, they do not directly prevent the
issuance of an altimeter anomaly. The alert software was developed to allow an observer the option to use
a “panic send” to transmit reports that might normally be considered suspicious (i.e. associated with
katabatic winds or severe thunderstorms) and all such reports are flagged for further review.
2.3
Identified Sources of Error
2.3.1
Each detected altimeter setting error was reviewed to determine the most probable cause
and contributing factors. The following conclusions were drawn:
a) observer typing errors and errors in the process of manually evaluating an altimeter
setting accounted for approximately 80% of all errors detected;
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AMOFSG/7-SN No. 23
b) a related finding, although not necessarily a causative factor, was that 70 percent of
altimeter setting errors were issued for smaller, less busy and often isolated, aerodromes;
c) some reports were sent with the wrong station identifier;
d) third party typing errors were sometime made when the report was transmitted by another
station;
e) mercury barometers were misread in some cases; and
f) an incorrect conversion process was sometimes followed to determine the altimeter
setting from ambient pressure.
2.3.2
Each case can be followed up with the personnel involved to ensure full understanding
of, and the ability to comply with, the procedures involved with particular emphasis especially in the case
of repeat individual errors.
2.4
Mitigation methods
2.4.1
It was recognized that proactive methods would have to be applied to reduce the incidents
of altimeter setting reports errors. Some of the available methods include:
a) implementing an operating guideline that it is better to suppress a suspicious
altimeter setting than to provide a potentially erroneous report;
b) introduction of new equipment that is less vulnerable to human factor errors
(automation and semi-automation) and which incorporate multiple pressure sensors
to allow for the immediate detection of a possible sensor failure;
c) improved performance requirements for pressure sensors including specifications
regarding siting, exposure, proper use of static ports (as necessary), etc.;
d) improved and amplified process documentation, and associated training, regarding
any human steps involved in the provision of altimeter setting reports;
e) enhanced and increased frequency of station inspections and preventive maintenance
routines and amplification of associated process documentation and training; and
f) in order to reduce the possible consequences of an altimeter setting error, notification
to pilots to provide an awareness of the possibility of erroneous altimeter setting
reports.
2.4.2
It should also be noted that exposure to the worst case scenario may be effectively
mitigated by the existence of “approach bans” that prohibit approaches in zero, or near zero, visibility.
2.5
Quality Assurance
2.5.1
It is notable that most, if not all, of the measures in 2.11 would be incorporated in
response to the introduction of a comprehensive quality management system. This would have associated
additional benefits to most aspects of aviation weather service provision.
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3.
CONCLUSIONS
3.1
It is proposed that the introduction of standards requiring comprehensive process
documentation and measures of compliance with procedures related, but not limited to, installation,
performance targets, siting, training, calibration and maintenance of meteorological services would
largely mitigate the hazards associated with potential altimeter setting errors in meteorological reports.
3.2
One specific means to achieve the intent of 2.14 would be to upgrade selected
recommendations from section 2.2 and paragraph 4.1.4 of Annex 3 to standards.
4.
4.1
ACTION BY THE GROUP
The AMOFSG is invited to:
a) agree with the safety critical nature of altimeter setting reports and the need to ensure
timely station inspections, the introduction of quality control and performance
monitoring and process management to mitigate the associated risks; and
b) agree with the conclusions stated in 2.14 and 2.15.
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AMOFSG/7-SN No. 23
Appendix
APPENDIX
80
Monthly altimeter Setting (QNH) anomalies of greater than 0.02 inches of mercury
equivlanet detected in Canadian METAR / SPECI
April 1999 - January 2008
70
50
40
30
20
10
0
Ap
r-9
Au 9
g9
D 9
ec
-9
Ap 9
r-0
Au 0
g0
D 0
ec
-0
Ap 0
r-0
Au 1
g0
D 1
ec
-0
Ap 1
r-0
Au 2
g0
D 2
ec
-0
Ap 2
r-0
Au 3
g0
D 3
ec
-0
Ap 3
r-0
Au 4
g0
D 4
ec
-0
Ap 4
r-0
Au 5
g0
D 5
ec
-0
Ap 5
r-0
Au 6
g0
D 6
ec
-0
Ap 6
r-0
Au 7
g0
D 7
ec
-0
7
Number of anoamlies
60
Month/Year
— END —