LJ35 crew misset the altimeter by 1 inch descending into bwi class east airspace.

Attributes
ACN	533020
Time
Date	200112
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : mrb.vortac
State Reference	WV
Altitude	msl bound lower : 15000 msl bound upper : 16000
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : iad.tracon
Operator	common carrier : air taxi
Make Model Name	Learjet 35
Operating Under FAR Part	Part 135
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 200 flight time total : 12000 flight time type : 9500
ASRS Report	533020
Person 2
Affiliation	company : air taxi
Function	flight crew : first officer
Events
Anomaly	altitude deviation : overshoot inflight encounter : weather inflight encounter other non adherence : company policies non adherence : far non adherence : published procedure other anomaly other
Independent Detector	atc equipment other atc equipment : radar other controllera
Resolutory Action	controller : issued alert flight crew : returned to assigned altitude none taken : detected after the fact
Supplementary
Problem Areas	Weather Flight Crew Human Performance FAA Aircraft
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

On descent to approach to bwi from en route environment, I asked the first officer for the altimeter setting. He responded '48.' I didn't clarify and set 30.48 inches. The correct setting was 29.48. Since the static defect correction module in the earlier lear jets does not correct the copilot's altimeter, the information provided there is largely ignored, except low and slow where the differences are minimal. In other words, both pilots look at the left side altimeter, which also corresponds to the mode C reply. Thus, we ended up 1000 ft low. Dulles pointed out the problem. We climbed back to assigned altitude. No conflicts reported. I should have demanded the full setting. My first officer should have crosschecked my setting with his (which was correct), and perhaps all learjets should have the rosemount static system so there can be a meaningful altitude xchk.

Original NASA ASRS Text

Title: LJ35 CREW MISSET THE ALTIMETER BY 1 INCH DSNDING INTO BWI CLASS E AIRSPACE.

Narrative: ON DSCNT TO APCH TO BWI FROM ENRTE ENVIRONMENT, I ASKED THE FO FOR THE ALTIMETER SETTING. HE RESPONDED '48.' I DIDN'T CLARIFY AND SET 30.48 INCHES. THE CORRECT SETTING WAS 29.48. SINCE THE STATIC DEFECT CORRECTION MODULE IN THE EARLIER LEAR JETS DOES NOT CORRECT THE COPLT'S ALTIMETER, THE INFO PROVIDED THERE IS LARGELY IGNORED, EXCEPT LOW AND SLOW WHERE THE DIFFERENCES ARE MINIMAL. IN OTHER WORDS, BOTH PLTS LOOK AT THE L SIDE ALTIMETER, WHICH ALSO CORRESPONDS TO THE MODE C REPLY. THUS, WE ENDED UP 1000 FT LOW. DULLES POINTED OUT THE PROB. WE CLBED BACK TO ASSIGNED ALT. NO CONFLICTS RPTED. I SHOULD HAVE DEMANDED THE FULL SETTING. MY FO SHOULD HAVE XCHKED MY SETTING WITH HIS (WHICH WAS CORRECT), AND PERHAPS ALL LEARJETS SHOULD HAVE THE ROSEMOUNT STATIC SYS SO THERE CAN BE A MEANINGFUL ALT XCHK.

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.