A319 crew set qnh before the transition altitude on descent to mmex.

Attributes
ACN	581363
Time
Date	200305
Day	Mon
Local Time Of Day	1201 To 1800
Place
State Reference	FO
Altitude	msl bound lower : 19600 msl bound upper : 20000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : mmex.artcc
Operator	common carrier : air carrier
Make Model Name	A319
Operating Under FAR Part	Part 121
Navigation In Use	other
Flight Phase	descent : intermediate altitude
Route In Use	arrival star : arrivals transition rwy 5l
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer pilot : cfi
Experience	flight time last 90 days : 75 flight time total : 9880 flight time type : 2900
ASRS Report	581363
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : company policies non adherence : far non adherence : published procedure other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : altimeter other flight crewa
Resolutory Action	controller : issued new clearance flight crew : returned to assigned altitude
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Descending into mexico city, we received a clearance to cross 10 NM north of positive control area at FL200. Once I saw we were easily going to make the restr, I made an arrival announcement on the PA system. When I finished, mexico center called us to 'maintain FL200 and contact mexico approach.' I noticed we were about 300-400 ft low. The captain had selected 30.32 inches in his altimeter and failed to return it to standard (29.92 inches). We began climbing, but approach cleared us immediately down to 16000 ft. No conflict was noted with other aircraft, nor did ATC comment further. I have noticed this behavior before. (Pre-selecting an altimeter setting and then failing to reset it to 29.92). I have done it myself in the past. With the new electronic-type altimeter, we can preset the altimeter and then push the button to return to standard. With older altimeters, this was not an option, and hence, not a problem. Maybe the 'standard' setting should not be available, forcing us to dial in 29.92, or the local setting.

Original NASA ASRS Text

Title: A319 CREW SET QNH BEFORE THE TRANSITION ALT ON DSCNT TO MMEX.

Narrative: DSNDING INTO MEXICO CITY, WE RECEIVED A CLRNC TO CROSS 10 NM N OF PCA AT FL200. ONCE I SAW WE WERE EASILY GOING TO MAKE THE RESTR, I MADE AN ARR ANNOUNCEMENT ON THE PA SYS. WHEN I FINISHED, MEXICO CTR CALLED US TO 'MAINTAIN FL200 AND CONTACT MEXICO APCH.' I NOTICED WE WERE ABOUT 300-400 FT LOW. THE CAPT HAD SELECTED 30.32 INCHES IN HIS ALTIMETER AND FAILED TO RETURN IT TO STANDARD (29.92 INCHES). WE BEGAN CLBING, BUT APCH CLRED US IMMEDIATELY DOWN TO 16000 FT. NO CONFLICT WAS NOTED WITH OTHER ACFT, NOR DID ATC COMMENT FURTHER. I HAVE NOTICED THIS BEHAVIOR BEFORE. (PRE-SELECTING AN ALTIMETER SETTING AND THEN FAILING TO RESET IT TO 29.92). I HAVE DONE IT MYSELF IN THE PAST. WITH THE NEW ELECTRONIC-TYPE ALTIMETER, WE CAN PRESET THE ALTIMETER AND THEN PUSH THE BUTTON TO RETURN TO STANDARD. WITH OLDER ALTIMETERS, THIS WAS NOT AN OPTION, AND HENCE, NOT A PROB. MAYBE THE 'STANDARD' SETTING SHOULD NOT BE AVAILABLE, FORCING US TO DIAL IN 29.92, OR THE LCL SETTING.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.