Premature setting of altimeter causes an altitude overshoot on descent and a TCASII RA.

Attributes
ACN	301790
Time
Date	199504
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : buf
State Reference	NY
Altitude	msl bound lower : 20700 msl bound upper : 21000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zny
Operator	common carrier : air carrier
Make Model Name	B737-200
Operating Under FAR Part	Part 121
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 210 flight time total : 12000
ASRS Report	301790
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : overshoot non adherence : clearance non adherence : far other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Miss Distance	vertical : 700
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While flying flight from lga to buf we were with ZNY flying direct buf level at FL240. ZNY gave us clearance to descend to 16000 ft with a local altimeter setting of 30.35, this clearance was received and the captain (the PF) started the descent and set his altimeter to 30.35 from 29.92. A min or so later as we were descending through FL230 ZNY called traffic to our attention at 2 O'clock and to stop the descent at FL210. This clearance change was received and the captain set FL210 in the autoplt altitude setting but forgot to reset his altimeter to 29.92. As we were looking for and spotting the traffic at 2 O'clock, we received altitude warnings and TCASII 'traffic' alerts. This brought our attention to the aircraft's altitude dropping approximately 300 ft below FL210. As the captain pulled back to bring the aircraft to FL210, a single RA warning was received from the TCASII. This altitude deviation was a result of premature setting of a local altimeter setting during descent, and was corrected without large evasive moves.

Original NASA ASRS Text

Title: PREMATURE SETTING OF ALTIMETER CAUSES AN ALT OVERSHOOT ON DSCNT AND A TCASII RA.

Narrative: WHILE FLYING FLT FROM LGA TO BUF WE WERE WITH ZNY FLYING DIRECT BUF LEVEL AT FL240. ZNY GAVE US CLRNC TO DSND TO 16000 FT WITH A LCL ALTIMETER SETTING OF 30.35, THIS CLRNC WAS RECEIVED AND THE CAPT (THE PF) STARTED THE DSCNT AND SET HIS ALTIMETER TO 30.35 FROM 29.92. A MIN OR SO LATER AS WE WERE DSNDING THROUGH FL230 ZNY CALLED TFC TO OUR ATTN AT 2 O'CLOCK AND TO STOP THE DSCNT AT FL210. THIS CLRNC CHANGE WAS RECEIVED AND THE CAPT SET FL210 IN THE AUTOPLT ALT SETTING BUT FORGOT TO RESET HIS ALTIMETER TO 29.92. AS WE WERE LOOKING FOR AND SPOTTING THE TFC AT 2 O'CLOCK, WE RECEIVED ALT WARNINGS AND TCASII 'TFC' ALERTS. THIS BROUGHT OUR ATTN TO THE ACFT'S ALT DROPPING APPROX 300 FT BELOW FL210. AS THE CAPT PULLED BACK TO BRING THE ACFT TO FL210, A SINGLE RA WARNING WAS RECEIVED FROM THE TCASII. THIS ALTDEV WAS A RESULT OF PREMATURE SETTING OF A LCL ALTIMETER SETTING DURING DSCNT, AND WAS CORRECTED WITHOUT LARGE EVASIVE MOVES.

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.