Pilot of a C172 descended below the teb VOR DME approach altitudes due to misreading the altimeter.

Attributes
ACN	480282
Time
Date	200007
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : teb.airport
State Reference	NJ
Altitude	msl single value : 500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : teb.tower
Operator	general aviation : personal
Make Model Name	Skyhawk 172/Cutlass 172
Operating Under FAR Part	Part 91
Navigation In Use	other vortac
Flight Phase	descent : approach
Route In Use	approach : instrument non precision
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : private
Experience	flight time last 90 days : 25 flight time total : 400 flight time type : 400
ASRS Report	480282
Person 2
Affiliation	government : faa
Function	controller : local
Qualification	controller : radar
Events
Anomaly	altitude deviation : overshoot non adherence : published procedure other spatial deviation
Independent Detector	atc equipment other atc equipment : radar aircraft equipment other aircraft equipment : altimeter
Resolutory Action	controller : issued alert 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:

I was northwest of teb outside of wanes intersection. Altitude was 3000 ft. The first step-down was from 3000 ft to 2500 ft. I am not sure why, but I thought I was at 4000 ft, so as I started the step-down I made a mental note that the hundreds hand would travel 1 1/2 revolutions before stopping on the '5.' since the step-downs were at a regular spaced interval and I was approaching faster than 90 KTS, my error probably was not apparent to the tower until I was past the last step-down and continued down. To the tower, it probably just looked like I was starting each step-down early. Warning signs that I missed were: 1) it was taking a lot longer to get down than I thought, 2) it seemed very low from the cockpit when I was at 700 ft. The tower questioned why I was not at 1500 ft. I told them I was at 1700 ft going to 1500 ft and they told me that I was at 700 ft and should check my altimeter. That is when I realized what was wrong. I told them I was correcting to 1500 ft and landed without event.

Original NASA ASRS Text

Title: PLT OF A C172 DSNDED BELOW THE TEB VOR DME APCH ALTS DUE TO MISREADING THE ALTIMETER.

Narrative: I WAS NW OF TEB OUTSIDE OF WANES INTXN. ALT WAS 3000 FT. THE FIRST STEP-DOWN WAS FROM 3000 FT TO 2500 FT. I AM NOT SURE WHY, BUT I THOUGHT I WAS AT 4000 FT, SO AS I STARTED THE STEP-DOWN I MADE A MENTAL NOTE THAT THE HUNDREDS HAND WOULD TRAVEL 1 1/2 REVOLUTIONS BEFORE STOPPING ON THE '5.' SINCE THE STEP-DOWNS WERE AT A REGULAR SPACED INTERVAL AND I WAS APCHING FASTER THAN 90 KTS, MY ERROR PROBABLY WAS NOT APPARENT TO THE TWR UNTIL I WAS PAST THE LAST STEP-DOWN AND CONTINUED DOWN. TO THE TWR, IT PROBABLY JUST LOOKED LIKE I WAS STARTING EACH STEP-DOWN EARLY. WARNING SIGNS THAT I MISSED WERE: 1) IT WAS TAKING A LOT LONGER TO GET DOWN THAN I THOUGHT, 2) IT SEEMED VERY LOW FROM THE COCKPIT WHEN I WAS AT 700 FT. THE TWR QUESTIONED WHY I WAS NOT AT 1500 FT. I TOLD THEM I WAS AT 1700 FT GOING TO 1500 FT AND THEY TOLD ME THAT I WAS AT 700 FT AND SHOULD CHK MY ALTIMETER. THAT IS WHEN I REALIZED WHAT WAS WRONG. I TOLD THEM I WAS CORRECTING TO 1500 FT AND LANDED WITHOUT EVENT.

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.