Issue Number 2 : October 1991
by Perry Thomas
|Also read these two sidebar articles...|
|The Question of Q's||What's a Pascal?|
The use of Hecto Pascal or Millibars by some countries has, on occasion, caused experienced international flight crews (who are accustomed to inches of mercury) to misset their aircraft altimeters.
The first officer was flying the approach and the captain called 1,000 feet MSL in descent. Shortly thereafter, the second officer called "300 feet radar altitude--go around!" A missed approach was flown and the "captain questioned the tower about altimeter setting 29.91 ...this was confirmed. A second voice, however, corrected that statement to 991 millibars" [emphasis added].
The aircraft's altimeters were reset from 29.91 to 991 millibars--a 640 foot difference. The flight crew later calculated they had come within 160 feet of hitting the water.
|What the flight crew SAW with the altimeter misset to 29.91"||But at 991 hectopascal, they were 120 feet below the MDA!|
The non-precision approach had an MDA of 420 feet. The LEFT altimeter graphic is what the flight crew saw with a misset altimeter.
The RIGHT altimeter graphic shows that they were actually 120 feet below the MDA at the point of the go-around. When executing a non-precision approach, it is common practice to use a higher rate of descent than for an ILS, thus, by the time that the aircraft's descent rate was arrested, they had actually descended as low as 160 feet above the surface!
Was this merely an isolated incident? Here is a second occurrence from the other side of the world.
It was the end of a long overwater flight...
Several human and procedural factors appear to increase the possibility of misset altimeters in international operations.
International flights from the United States are generally of long duration through several time zones. The element of fatigue in long distance flights is inescapable.
Workload on Approach
Transition from standard altimeter setting flight levels (QNE) to sea level altimeter setting altitudes (QNH) are generally much lower than in the United States. Obtaining altimeter settings and landing data closer to the approach segment complicates the task of preparing data for landing at the very time the flight crew may be most fatigued.
Rapid delivery of clearances coupled with unfamiliar accents, and contraction of hPa (hectopascal) or mb (millibars) increase the potential for error. This also must be true of flights arriving in the United States from other countries. Other flight crews communicating in their native tongues contribute to a lack of awareness of what other traffic is doing.
Only one person receiving the approach and landing data and passing that information to the rest of the crew means that a misconception or misunderstanding is less likely to be detected until too late.
There is often inadequate crew briefing for approach and landing with no mention of how the altimeter setting will be expressed--that is, Hg, mb, or hPa. Flight crews also may not adequately review approach charts for information. Some airlines do not provide the second officer with approach plates; unless he or she makes an extra effort to look at one of the pilot's charts, the altimeter setting standard may be unknown.
Experience Level and Currency
At least one airline experiences a constant turnover in the international group as senior pilots retire and other crew members bid off international schedules to upgrade to captain or first officer. Many of the international reports submitted to ASRS mention that at least one flight crew member is new to the operation. Airline training is usually reported as being adequate, but some of the training for international operations may not be used or need to be recalled for months after the training is received.
Some of the aspects involved, such as fatigue, will be more difficult to overcome. Implementing sterile cockpit procedures, avoiding distractions during periods of high cockpit workload, and getting adequate crew rest and nourishment will help to avoid those famous last words...
We all tend to forget things we either have not used in a while, or we don't use very often. For those of us who need a memory refresher, here are three important "Q" altimeter settings:
QNE: The standard altimeter setting of 29.92 inches of mercury (the contraction is Hg.), or 1013.25 hectopascal (hPa for short), or 1013.25 millibars (use mb). See the sidebar on the next page titled "What's a Pascal?" to find out why hPa and mb are the same.
QNH: Height above sea level when corrections are applied for local atmospheric pressure that is above or below the standard altimeter setting of 29.92 Hg. QNH is the altimeter setting provided in the ATIS information and by ATC.
QFE: An altimeter setting that is corrected for actual height above sea level and local pressure variations
The "Q" codes referred to here may be found in the Tables and Codes section of the Jeppesen Sanderson airways manuals.
The term "hectopascal" is derived partly from the name of a 17th century philosopher and mathematician, and partly from the Greek.
Blaise Pascal was born in 1623 in France. A youthful genius in mathematics, at age 21 he developed and built the first digital computer. Pascal's Law of Pressure was developed in 1647 and is the principle that created hydraulic lifts, and eventually the hydraulic brakes in our automobiles. Using Evangelista Torricelli's work on the principle of the barometer, Pascal developed his own method of measuring barometric pressure.
Hecto is an irregular contraction of the Greek word for hundred from the metric system of measurement--hence hecto-pascal, often abbreviated to HP or hPa. In common usage, one hPa equals one millibar.
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