by Marcia Patten and Ed Arri

"This was the last leg of a long 3-day trip...Inbound...we ran the 'preliminary checklist,' cross-checking altimeters at 30.22. This seemed a little odd to me at the time as the area had a low front moving through, but we were busy and I did not press the issue. Once on approach, everything was normal until just before the final approach fix when we broke out of the clouds and a ridge was looking very close. Also the GPWS went off as we passed over the ridge. I checked our altitude and we were right on profile. I had the Captain check the altimeter with Tower. Altimeter was actually 29.22, not 30.22, putting us approximately 1,000 feet too low on approach." (# 292718)

The incident cited above is one of many typical altimeter-missetting reports sent to the Aviation Safety Reporting System (ASRS). ASRS analysts note that these reports often come in bunches, as numerous flight crews experience the same problem on the same day in a particular area that is encountering unusual barometric pressures. Incorrect altimeter settings are a direct cause of altitude deviations, some of them severe enough to result in near mid-air collisions and controlled flight toward terrain. Fortunately, most of these deviations are detected following an alert from ATC, GPWS, or TCAS, and are corrected before the situations become truly perilous.

In this article, we present some of the common scenarios for altimeter missetting incidents. In particular, we focus on incidents associated with the very low altimeter settings that often occur during the winter months. We also consider the influence of human behavior in altimeter-missetting incidents, and offer suggestions for pilots to avoid falling prey to falling barometric pressure.

Where in the World...?

We searched the ASRS database for altimeter-missetting incidents that occurred during extremely low barometric pressures, and found reports from far and wide. We also enlisted the aid of the Canadian Aviation Safety Board, and obtained some of their reports of incidents and accidents attributed to misset altimeters.

The greatest number of reports referenced far-north or very cold locations-many incidents occurred in Alaska and in cities near the Great Lakes. Other locations known for severe weather and cold temperatures were also well-represented - New York, the high-altitude Rocky Mountains, and parts of Canada. Somewhat surprisingly, there were also reports from otherwise relatively temperate locations, such as San Francisco, California; Portland, Oregon; Kansas City, Kansas; and Richmond, Virginia. Although these areas generally have less severe weather, unusual frontal systems created some significant changes in barometric pressure and caught several reporters off-guard.

There were also a number of reports of incidents that occurred in foreign locales-Moscow, Keflavik, Copenhagen, Frankfurt, Brussels. Many of these locations are even farther north, that is, at higher latitudes, than the locations in the domestic incident reports.

Weather or Not

Weather plays a significant role in many incidents of misset altimeters. A semi-permanent low pressure area off the Aleutian Islands is the perfect set-up to bring frequent low barometric pressures to Alaska and Western Canada. Likewise, a winter-season low that forms between Greenland and Iceland provides very low altimeter settings across those areas and in Eastern Canada.

The fierce cold fronts that race through central Canada and the north-central and northeast sections of the United States often have steep frontal slopes, resulting in rapid lifting movement of air. This movement causes sudden drops in barometric pressure. After frontal passage, the barometric pressure rises again. During these rapid ups and downs, a number of pilots found themselves missetting their altimeters. A General Aviation pilot reports:

• "I set the field elevation on the altimeter...and departed VFR, [then] contacted Center and received my clearance. In the climb I encountered icing conditions and...I must have misset the altimeter. A frontal passage was in progress and the barometer was changing rapidly. Center told me to stop squawking altitude, as my transponder and altimeter did not agree. Then I suddenly was surprised to realize I had set my altimeter wrong at 30.82 instead of 29.82." (#293162)

A helicopter accident resulting in four fatalities was attributed at least in part to an incorrectly set altimeter during a period of known low barometric pressure. The report from the Canadian Aviation Safety Board states:

• "The helicopter was being used to transport personnel to work sites across a large frozen lake. An approaching low pressure area with snow and high winds...reduced visibility to near zero in some areas. The pilot most certainly encountered adverse conditions and altered course to circumvent the worst areas. The aircraft was later found...wreckage was widely scattered. The altimeter showed a setting on impact of 30.05; the correct setting would be about 29.22, causing the altimeter to read about 800-850 feet high. The altimeter had obviously been set two days previously [apparently during a time of high barometric pressure-Ed.]." (A80C0002)

Weather or Not you are flying near significant weather activity, ensure that you have the most up-to-date weather reports, including complete information on frontal movement.

Barometer Surprise

A number of reporters indicated that the low altimeter setting they encountered was unusual for the area or unexpected for the current weather conditions. In many of these cases, the crew subconsciously ignored the correct setting in favor of a setting that seemed more appropriate. Some examples:

• "The altimeter setting I wrote down was 28.85, but we had both set 29.85. I did not recognize the unusual nature of the setting, and reverted to more familiar settings during the checklist." (#97654)
  
  
• "I read back the clearance, understanding the altimeter to be 30.37. Factors in this incident include my hearing ".37" and assuming it was the more normal 30.37 rather than the [actual] low reading of 29.37." (#287167)
  
  
• "Altimeter was 29.32. First Officer set his altimeter to 29.32; I did not check mine. Weather was such that a 29.32 altimeter setting was not expected-winds were calm and a clearing trend was in the works." (#295359)

Crews can avoid a Barometer Surprise by listening carefully to ATIS and ATC broadcasts, especially before, during and after significant weather, when the altimeter setting may be an unusual number.

Et cetera

Our research produced one other oddity associated with a low altimeter setting:

• "The temperature was 53 degrees with an altimeter setting of 29.34. I advised the Captain that we were over our allowable takeoff gross weight for that runway. The low altimeter setting required a further reduction of 270 pounds in allowable takeoff weight. The Captain said he was not aware of correction due to altimeter settings. The procedures for adjusting weights...had been in effect for only a few months. Before this, altimeter settings had only affected takeoff power, not takeoff weights. A test or some classroom work should be sufficient to correct the situation." (#145629)

This report serves as a reminder for all pilots to review flight and operations manuals frequently for changes or additions to out-of-the-ordinary procedures.

The Far Side

FAR 91.121 states that, when operating an aircraft below 18,000 feet MSL, pilots will maintain altitude by reference to an altimeter that is set to: 1) the current reported altimeter setting of a station along the route of flight and within 100 nautical miles of the aircraft; 2) the current reported altimeter setting of an appropriate available station; or, 3) in aircraft without a radio, the elevation of the departure airport or an appropriate setting available before departure.

This is not a problem on most flights. However, some routes or operations may take a pilot far from an altimeter reporting station, as was the case with this General Aviation pilot:

• "Southeast bound [on airway] at 17,000 feet indicated altitude, Controller reported my altitude encoder indicated 16,000 feet on the readout. I had departed VFR and picked up my IFR clearance at about 4,000 feet... I had set the barometric pressure as provided by Center when clearance was provided. I was approaching a cold front which was lying north to south over Lake Michigan. I asked for an altimeter setting. The setting provided was 1 inch lower than the previous provided setting (about 100 nm earlier). I reset my altimeter... After the reset, my altimeter now indicated 16,000 feet... The problem was evidently a very steep pressure gradient behind the cold front." (#190851)

Large portions of the Canadian provinces and territories are remote, making aircraft an ideal form of transportation to and from these far-off areas. However, flying in such remote locations is not without some hazards. Two incident reports from Canada provide graphic examples of why an accurate altimeter setting can be critical:

• "The pilot stated that he was lined up [on approach] for the runway and that the altimeter was reading 300 feet when the nose wheel struck the ice. The pilot applied full power...and flew back to [his departure airport over 100 nm south-Ed.], where the landing was uneventful. The pressure in the area was lower than the point of departure, sufficient to make the altimeter read 250-300 feet high if not properly reset." (A80W0001)
  
  
• "On a night VFR flight, the pilot encountered deteriorating weather as he approached his destination. He received an IFR clearance...During a procedure turn, the aircraft started to strike the tree tops. The aircraft stalled and crashed into the trees. Because the airport had been closed for the night, no altimeter setting was available. The FSS operator gave the pilot the setting for XYZ (29.68) and for ABC (29.87) [approximately 90 nm south and 90 nm east, respectively-Ed.]. The aircraft altimeter was set at 29.94. Both pilots had been without proper rest for approximately 20 hours." (A80C0079)

To avoid being left on the Far Side, obtain altimeter settings from the nearest FSS or ATC facility. Then give yourself an extra margin for error when flying or landing in areas far from the altimeter-reporting station. Remember the old adage, "High to low, look out below." A one-inch error in the altimeter setting equals 1,000 feet of altitude.

The One-Eight-Zero Blues

ASRS receives many reports of altimeter-missetting incidents that occur when aircraft are transitioning through FL180 (see "Transitioning Through FL 180". A flight crew's failure to reset an altimeter at FL180 has probably caused an occasional adrenaline rush among controllers; the added factor of an extremely low barometric pressure increases the potential for large altitude errors. The following report excerpts illustrate:

• "Altimeter [at departure field] 28.42. When...climbing through 18,000 feet, Captain called, '29.92 set' when in fact he set 28.92. I did the same. The back lighting in my altimeter was out, and maintenance had installed post lights [which left] a dark shadow...thus my meter was in the dark. I was careless in not double-checking with a flashlight." (#290765)
  
  
• "Holding off and on. We neglected to reset altimeters from 29.92 to 29.20 passing through FL180. Extremely low pressure caused us to be at 12,200 feet when we thought we were at 13,000 feet. We didn't accomplish the checklist on descent, which would have prevented this." (#289818)
  
  
• "Received low altitude warning, pulled up and discovered altimeter was misset. Altimeter was set at 29.84, and should have been set at 28.84. Crew distracted with a [mechanical problem] about the time of altimeter transition from flight levels to altitudes." (#290122)

The cure for the One-Eight-Zero Blues is strict adherence to checklists and procedures (sterile cockpit, readback of ATC clearances, etc.), and good CRM techniques for cross-checking with the other crewmember(s).

"Bar" Exam

Hectopascals, more commonly referred to as millibars, are used in many foreign countries instead of inches of mercury as the unit of measurement of barometric pressure. (More information on International Altimetry can be found in ASRS Directline #2, Fall, 1991). Reporters noted that distractions or inattention to details were precursors to many of the incidents that occurred where millibars are the status quo. Others pointed to a lack of clear communication, as the next reporter suggests:

• "During descent, the altimeters were incorrectly set at 29.99 instead of 0999 hectopascals, resulting in Approach Control issuing an altitude alert. I believe the ATIS was copied by the relief pilot using 3 digits with a decimal point. Since [ATIS] normally issues both hectopascals and inches of mercury, I incorrectly assumed that the decimal denoted the inches-of-mercury scale and announced '29.99,' and set my altimeter. I recommend the following phraseology: 'Altimeter zero 999 millibars' for hectopascal scale; 'Altimeter 2999 inches' for inches-of-mercury scale." (#295007)

Even when you know you are working in millibars, just how few millibars can come as a surprise:

• "Altimeter setting 971 mb [descending] out of 4,000 feet (transition altitude) ...started to set altimeter. By the time I had set 971 mb, I was 500 feet below assigned altitude. Approach Control noticed what had happened and cleared us to 2,000 feet...and said, 'Altimeter setting is pretty low, huh!' Never having used millibars before, the significance of 971 mb wasn't apparent to me until I read the inches of mercury equivalent, 28.68." (#101698)

Pass your Bar Exam in foreign countries; be particularly vigilant where altimeter settings may be in units other than inches of mercury, and where altimeter transition levels, from pressure altitude (QNH) to the standard pressure setting of 29.92 (QNE), and vice-versa, may be variable.

Feeling Pressured?

Other reporters confessed to simple human error-mistakes in reading, hearing, or copying the broadcast altimeter setting; distractions and inattention; and failure to complete checklists.

• "The 30.06 altimeter setting we used was actually the wind speed and direction and was written [on the ATIS information card as] 3006. In my mind, this was a reasonable altimeter setting. The ATIS setting was actually 29.54." (#292949)
  
  
• "PNF understood ATIS recording to state altimeter setting to be 29.99 when actually the setting was 29.29." (#293372)
  
  
• "First flight of the day after overnight maintenance...and there was adequate time to accomplish all required checks. Maintenance had set the Captain's altimeter to zero. Departure field elevation was almost exactly 1,000 feet MSL. This was a very subtle trap, but we had a lot of chances to catch it." (#300270)

Even with what appeared to be a clear reminder for the flight crew, this Second Officer reports that they all still missed the "heads-up:"

• "The altimeter was 28.84. I remember enlarging the 8's with two circles on top of each other, thinking this would be sufficient in drawing attention to the low altimeter setting. The next crew after our flight...found the altimeter to be set at 29.84 instead of the actual 28.84 setting." (#195014)

Take the Pressure off by applying solid CRM skills. Keep all crew in the loop and confirm communications (verbal and written) with each other.

ATC's Role

A number of reporters expressed concern that ATC had "failed" to warn the flight crews about unusually low altimeter settings. However, misconceptions abound regarding ATC or FSS personnel's responsibility during low pressure situations. Two report excerpts illustrate:

• "ATIS [reported altimeter] 28.84. No mention of low altimeter was made. [Climbing through] FL180, altimeters were set to 28.92 Captain and 29.92 First Officer. At FL320 Captain's altimeter, I called FL320 for 330. The First Officer called 330, noticing the wrong altimeter setting on my side. I immediately descended...while setting my altimeter to 29.92. I feel this mistake might have been avoided if the ATIS had mentioned the low altimeter setting." (#290458)

In recording the ATIS, some controllers may emphasize the altimeter setting by stating, for example, "a low 28.84." However, this procedure is not mandatory.

• "Destination weather [reported altimeter] 28.83. Prior to initial descent, the Second Officer received and put the ATIS information on the landing bug card, except that the altimeter was written as 29.83... [On final], the Captain started a go-around at the same time the Tower reported they had a low altitude alert warning from us... I feel anytime [the altimeter setting] is below 29.00, the term 'low/low' should be used." (#290848)

Again, there is no requirement for controllers to notify pilots of unusually low barometric conditions, although many controllers elect to do so. The phrasing "low/low" is a technique used by some controllers and Flight Service Station specialists to emphasize a particularly low altimeter setting, but pilots shouldn't count on hearing it.

As little as a year ago, the FAA Air Traffic Procedures Division again looked into the suggestion that controllers state the word "low" before issuing an altimeter setting below 29.00 inches. Ultimately, the proposal was not adopted. In explaining the decision, the FAA stated in part: "The low altimeter issue has been determined to be geographically specific. A Regional or facility directive would be most effective in this case. The [automated ATC systems] can be adapted...to alert the air traffic control personnel to emphasize an unusual situation."

Summary

So, Wherever in the World you fly, avoid Feeling Pressured by a Barometer Surprise or the One-Eight-Zero Blues. Weather or Not you pass your Bar Exam, learn ATC's Role before you reach The Far Side.

Obtain frequent and appropriate weather reports throughout the flight. Listen carefully to the complete ATIS or ATC altimeter-setting broadcast, and confirm the information with other crewmembers.

High to Low, Hot to Cold

Flying into cold air has the same effect as flying into a low pressure area, that is, the aircraft is lower than the altimeter indicates. Unfortunately, altimeters cannot be corrected for temperature-related errors. However, pilots can adjust their minimum procedure altitudes to compensate for temperature errors. Canadian pilots consult a government-provided chart to determine how much altitude to add to the published procedure altitudes listed on approach charts, thereby ensuring obstacle clearance when temperatures are extremely low. The U.S. Defense Mapping Agency also publishes an altitude correction table, which is available to military pilots.

Another helicopter accident report from the Canadian Aviation Safety Board points out the hazards of failing to correct for temperature. Fortunately, there were no fatalities in this incident:

• "The helicopter was dispatched [to an offshore oil rig inside the Arctic Circle] at night, in IFR conditions... The crew descended on the inbound leg to 150 feet, with reference to the pilot's altimeter. The helicopter struck the sea ice and was destroyed by post-impact fire. The crew had not applied a temperature correction to the minimum descent altitude [approximately 40 feet to as much as 100 feet-Ed.], and this omission-combined with the known 50-foot error in the pilot's altimeter-accounted for the mistaken belief the helicopter was higher." (A81W0134)

For those who have never used an altitude correction chart, here is an example of how the Canadian chart works. The Whitehorse airport, in the Yukon Territory along the Alaska-Canada highway, is approximately 2,300 feet MSL elevation. The approach plates indicate, "Mountainous terrain all quadrants. Apply altitude corrections for cold temperatures."

At the 10-mile marker, for example, the published crossing altitude is 6,600 feet MSL. On a -30 degreesC day (-22 degrees F) on the ground, pilots would add more than 700 feet to that altitude; that is, they would cross the fix at an indicated altitude of 7,300 feet MSL to offset the error caused by the cold temperature and to ensure obstacle clearance. At the final approach fix, the published altitude of 4,000 feet MSL would need to be increased approximately 300 feet; that is, pilots would cross the fix at 4,300 feet MSL indicated altitude. In other examples, at higher published altitudes and at colder surface temperatures, corrections can be more than 1,000 feet-a potentially critical difference between true altitude and indicated altitude, especially in IMC.

Transitioning Through FL180

FL180 is the altitude at or above which, in North America, all aircraft altimeters should be set at 29.92, and below which they should be set to the current barometric pressure of the nearest reporting station. Extreme barometric pressure is only one of the causes reporters cited for the altimeter-missetting incidents that occurred during a climb or descent through this altitude. A frequently reported cause was distraction by other cockpit tasks. Other causes noted by ASRS analysts were failure to follow procedures and lack of Crew Resource Management (CRM) skills.

All three of the following report excerpts indicate a lack of CRM, and a resultant failure to maintain an adequate division of labor among the cockpit crew. In the first report, numerous distractions inside and outside the cockpit, combined with an apparently uncompleted checklist, led to a relatively minor altitude deviation:

• "Reported weather was thunderstorms and hail. We were on a heading and altitude...that kept us parallel to a line of thunderstorms. After level-off at FL290, [the Center Controller] called us 500 feet high. In all the confusion...we neglected to reset altimeters at FL180. The problem arose...during a high workload period of time, a period of moderate turbulence, lightning nearby, working with airborne radar to determine our safest flight path, and communicating constantly with the Controller." (#107888)

The next reporter likewise experienced high workload and multiple distractions, including a minor mechanical malfunction.

• "Descending through approximately 23,000 feet and while navigating an area of precipitation and thunderstorms, both air conditioning packs failed. ...as we worked on the pressurization problem...we were assigned 11,000 feet. As we leveled, ATC asked our altitude because he saw us at approximately 10,500 feet. Then we noticed that two of our altimeters were still set at 29.92 with the [actual] pressure at 29.42. Our workload was obviously heavy, but we should not have missed this basic procedure." (#265215)

Again, appropriate division of cockpit tasks (one pilot to fly the aircraft, the other to handle the malfunction), and adherence to procedure (the checklist) probably would have caught this mistake before ATC did. At the very worst, left unnoticed, this incident had the makings of a repeat of other distraction-related accidents.

Another distraction, in the form of food, was the undoing of the next reporter:

• "Just before we began descent, the flight attendant brought up dinner for both of us at the same time. Started descent as we started eating. Because of distraction, we failed to reset altimeters at 18,000 feet. Descended to 17,000 feet with wrong altimeter setting. Received TA of traffic at 16,000 feet." (#295619)

Many air carriers have established policies that forbid the Captain and First Officer eating meals at the same time.

Transitioning Elsewhere

Beyond the North American continent, the pressure altitude/indicated altitude transition level is variable. In South America, Buenos Aires, Argentina is at the low end at 3,000 feet; the high end is 18,000 feet in La Paz, Bolivia. Most of Europe uses 4,000-6,000 feet; much of India also uses 4,000-5,000 feet. The transition level in Tel Aviv, Israel is 10,500 feet, but Jerusalem's transition altitude is changed by ATC as required. Cape Town, South Africa uses 7,500 feet, and further north, Cairo, Egypt uses 4,500 feet. To the East, in Riyadh, Saudi Arabia, the transition level is 13,000 feet. Australia uses 10,000 feet; Japan uses 14,000 feet; much of the rest of the Far East uses 11,000 feet. Above these transition levels, altitude is expressed as "Flight Level" (FL), and altimeters will be set to QNE-the standard pressure setting of 29.92 inches of mercury, or 1013.25 hectopascal.

In the following report from a flight crew on a European flight, the unfamiliar, non-standard transition altitude simply added to the distractions of the departure workload.

• "Climbing to FL60 (transition altitude 4,500 feet)...We were task saturated flying the Standard Instrument Departure, reconfiguring flaps and slats, resetting navigation receivers and course settings, resetting engine anti-ice, etc. The crew missed resetting the Kohlsman window to 29.92 at 4,500 feet MSL, and leveled off at FL60 indicated altitude with a Kohlsman setting of 28.88 inches. Departure informed us of our error." (#206218)

It would have been easy for this three-person crew to unconsciously think, "We'll get all this other stuff taken care of, then change the altimeter at FL180." Again, it was ATC to the rescue, bringing the problem to the crew's attention before the error became critical.

Thanks, eh?

Many of the reports used in this article were provided by ASRS's sister agency in Canada, the Canadian Aviation Safety Reporting Program (CASRP). Our thanks to Les East of the CASRP for helping us find so many useful incident records.

The CASRP incident reports may be identified by their combined letters and numbers (A80C0079), while ASRS incident reports use only numbers (#290458).