Issue Number 233
November 1998
A Monthly Safety Bulletin from The Office of the NASA Aviation Safety Reporting System
P.O. Box 189, Moffett Field, CA 94035-0189

Altimeter Settings Revisited

FL180 is the altitude at or above which 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. A frequently reported cause for altimeter mis-setting incidents that occur during a climb or descent through this altitude is distraction by other cockpit tasks. In a report to ASRS from an air carrier Captain, distractions inside and outside the cockpit, including a mechanical malfunction, led to an altitude deviation:

A 1997 ASRS study on flight crew monitoring incidents found that a large majority of such incidents occurred when the aircraft was in a "vertical" flight mode -- climbing or descending. Flight crews also were more likely to experience monitoring errors while performing two or more flight-related tasks -- like the crew in this report who were avoiding weather, dealing with a pressurization problem, and talking to ATC, all while descending through FL180.

As our reporter noted, appropriate division of cockpit tasks (one pilot to fly the aircraft, the other to handle the malfunction), and adherence to procedures (the checklist) probably would have allowed the flight crew to catch this mistake before ATC did.

12 O’Clock High

An air carrier crew's altitude problem started during preflight, when they failed to notice that their altimeter needles were aligned at the "12 o'clock" position -- at an airport with a field elevation of 1,000 feet MSL. The First Officer reports:

Aircraft AltimeterHigh to Low, Look Out Below

The rapidly changing weather associated with cold fronts and steep frontal slopes can create significant and sudden drops in barometric pressure, causing some pilots to mis-set their altimeters. An air carrier Captain provides an example:

Unusually low barometric pressures may take pilots by surprise, especially if the weather appears to be improving, leading the crew to believe that a higher altimeter setting looks plausible. The old adage, "High to low, look out below" is still sound advice.

With winter around the corner, a related reminder applies: 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. Altimeters cannot be corrected for temperature-related errors. However, pilots can adjust their minimum procedure altitudes to compensate for extremely low temperatures. Canadian pilots consult a government-provided chart to determine how much altitude to add to the procedure altitudes listed on approach charts, thus ensuring obstacle clearance during very low temperature operations. The U.S. Defense Mapping Agency publishes a similar altitude correction table for military pilots.

Readers who would like more information about low temperature correction charts should refer to ASRS Directline, Issue #9, available on the ASRS Web site, at [Actually, just click on this hyper link to go directly to The Low-Down on Altimeter Settings by Marcia Patten.-Ed]

To Enter "B" or Not to Enter "B"...

Many pilots assume that VFR "flight following" offers more ATC services than it does. A General Aviation pilot reports entering Class B airspace without a clearance, after mistakenly believing that VFR flight following service would provide the necessary clearance.

VFR flight following provides traffic advisories, not clearances or traffic separation, and only as controller workload permits. Pilots are responsible for monitoring their position and making a timely request for clearance into the Class B area. Often, the controller providing traffic advisories can coordinate the issuance of a clearance upon request from the pilot.

In the next incident, ATC was trying to provide advisories, but an apparent malfunction in the aircraft radio interfered with the controller's efforts, nearly putting the reporter in harm's way.

The reporter used LORAN as the primary navigation source, and relied on ATC for "back-up." Making full use of all resources, including charts and other navigational aids, will help keep pilots out of "hot" areas.

Traensponder with Code 7500 Dialed InTransponder Transgressions

An aircraft that squawks an incorrect transponder code can cause a lot of confusion for ATC. Squawking an incorrect emergency transponder code can also cause major embarrassment for the flight crew -- and added workload for others involved in the miscommunication -- as the next report suggests:

One memory trick for ensuring the correct use of 7500 is to think of the hand-slapping gesture "high-five," often signifying a job well done. Pilots will earn a high-five if they correctly remember that only a hijack warrants a 7500 transponder code.

ATC 1, Flight Crew 0000

In another report of an incorrect transponder code, ATC won game, set and match. The "losing team" tells the story:

Some flight crews make a habit of setting all zeroes in the transponder when they depart the aircraft, as a heads-up to the next crew to obtain their clearance and new squawk code. The outbound flight crews can improve their "score" by using the checklist item for transponder "on" as a reminder to ensure that the correct squawk code is "in."

ASRS Recently Issued Alerts On...
Uncommanded autopilot disengagement on a DA-20
K-100 fuel leak attributed to a dislodged fuel vent line
Pilot confusion over control of stop-bar lights at a Florida airport
Inadequate runway signs/markings reported at a Nevada airport
Uncommanded reduction of B757-200 engine power to flight idle
September 1998 Report Intake
 Air Carrier/Air Taxi Pilots 1,680
 General Aviation Pilots 697
 Controllers 59
 Cabin/Mechanics/Military/Other 198
 TOTAL 2,634