by Perry Thomas

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.

Europe

• "[A] three-man, wide-body type aircraft flight crew experienced in European operations" was engaged in a difficult (9 degrees drift over water in heavy rain) VOR­DME approach to an MDA of 420 feet. The transition altitude had been 4,000 feet so the experienced, but weary, flight crew was late receiving ATIS, reducing the time available for completing their landing data. QNH was given as nine-nine-one.

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.

Was this merely an isolated incident? Here is a second occurrence from the other side of the world.

The Orient

It was the end of a long overwater flight...

• "Approach control gave the altimeter as 998 hectopascal. I read back 29.98. [The] approach controller repeated his original statement. Forgetting that our altimeters have settings for millibars and hectopascal (which I had only used once in my career, and that was 6 months ago), I asked where the conversion chart was. 'Old hand' captain told me that approach [control] meant 29.98. Assuming that he knew what he was doing, I believed him. We were a bit low on a ragged approach and I knew we were awfully close to some of the hills that dot the area...but it was not until we landed and our altimeters read 500 feet low that I realized what had happened."

Quotes from Other ASRS Reports

• "Never having used mb before, the significance of 971 mb wasn't apparent to me until I read the equivalent Hg 28.68."
  
  
• "Dealing in millibars did not make an impression...[because of] the very low [atmospheric] pressure."
  
  
• "The copilot who had copied the ATIS gave me 29.97 when I asked for QNH. Gusty winds and [the controllers] thick...accent weren't helping things. [Obstructions] seemed unusually close to our altitude. [The] copilot had assumed 9-9-7 to be 29.97. " (500 feet low.)
  
  
• "[Given] altimeter of niner-seven-eight hPa. The hPa was somewhat muted. We set 29.78 [inches]." (900 feet low.)

Factors

Several human and procedural factors appear to increase the possibility of misset altimeters in international operations.

Fatigue

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.

Language Difficulties

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.

Communication Procedure

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.

Cockpit Management

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.

Recommendations

• Review approach charts prior to the descent, approach and landing phase. Each flight crew member should pay particular attention to whether altimeter settings will be given in inches (Hg), millibars (mb), or hectopascal (hPa).
  
  
• Use precise radio phraseology; confirm with ATC any radio communication that is not fully understood. Radio phraseology considered standard in one country may not be accepted or understood in another.
  
  
• Keep more than one flight crew member in the communications loop--including ATC clearances and ATIS messages.
  
  
• Practice good cockpit management technique. Include in the approach briefing how the altimeter setting will be expressed.
  
  
• Observe proper crew coordination. Flight crews need to crosscheck each other for accurate communication and procedure. Question anything that does not seem right.

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...

...I ASSUMED.

The Question of Q's

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.

• ON THE GROUND--a variable elevation reading that is above or below actual elevation (unless the station pressure happens to equal 29.92 Hg).
• IN THE AIR--positive separation by pressure level, but at varying actual or true altitudes.

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.

• ON THE GROUND--the actual elevation above sea level when the aircraft is on the ground.
• IN THE AIR--the true height above sea level (without consideration of temperature).

QFE: An altimeter setting that is corrected for actual height above sea level and local pressure variations

• ON THE GROUND--zero elevation when the aircraft is on ground. Thus, for an aircraft at the gate at Denver (actual airport elevation above sea level­5333 feet), the aircraft altimeters would read zero if set to QFE.
• IN THE AIR--the height above ground (without consideration of temperature).

The "Q" codes referred to here may be found in the Tables and Codes section of the Jeppesen Sanderson airways manuals.

What's a Pascal?

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.