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One of 2 light transport xponders gives erroneous altitude readout to ATC causing ATC to believe that the flight crew is not at assigned altitude.
| Attributes | |
| ACN | 316030 |
| Time | |
| Date | 199509 |
| Day | Sun |
| Local Time Of Day | 0601 To 1200 |
| Place | |
| Locale Reference | atc facility : aml airport : iad |
| State Reference | VA |
| Altitude | msl bound lower : 4000 msl bound upper : 4000 |
| Environment | |
| Flight Conditions | Mixed |
| Light | Daylight |
| Aircraft 1 | |
| Controlling Facilities | tracon : iad |
| Operator | common carrier : air carrier |
| Make Model Name | Jetstream 32 |
| Operating Under FAR Part | Part 135 |
| Flight Phase | cruise other descent other |
| Route In Use | enroute : direct enroute : on vectors enroute airway : iad |
| Flight Plan | IFR |
| Person 1 | |
| Affiliation | company : air carrier |
| Function | flight crew : first officer |
| Qualification | pilot : commercial pilot : instrument pilot : cfi |
| Experience | flight time last 90 days : 280 flight time total : 2000 flight time type : 375 |
| ASRS Report | 316030 |
| Person 2 | |
| Affiliation | company : air carrier |
| Function | flight crew : captain oversight : pic |
| Qualification | pilot : atp |
| Events | |
| Anomaly | aircraft equipment problem : less severe other anomaly other |
| Independent Detector | atc equipment other atc equipment : unspecified other controllera other flight crewa |
| Resolutory Action | flight crew : overcame equipment problem other |
| Consequence | faa : reviewed incident with flight crew |
| Supplementary | |
| Primary Problem | Aircraft |
| Air Traffic Incident | other |
Narrative:
About 25 mins into the flight the captain lost his intercom in cruise flight at 9000 ft. This was no factor to the flight since we did re-establish 2-WAY communication between us. We received a descent to 4000 ft where we leveled off and were handed off to dulles approach from ZDC. Dulles approach then confirmed 4000 ft and to depart barin intersection on 320 degree heading to intercept the runway 1L localizer when we checked in. After departing barin intersection, on a 320 degree heading dulles approach asked us our assigned altitude. I replied 4000 ft. He said he was showing us at 3500 ft and asked us our altitude. I told him we were level at 4000 ft. That's all that was said. The captain at that time switched to the second transponder and asked ATC what we were showing on his radar. ATC replied 4000 ft.
Original NASA ASRS Text
Title: ONE OF 2 LTT XPONDERS GIVES ERRONEOUS ALT READOUT TO ATC CAUSING ATC TO BELIEVE THAT THE FLC IS NOT AT ASSIGNED ALT.
Narrative: ABOUT 25 MINS INTO THE FLT THE CAPT LOST HIS INTERCOM IN CRUISE FLT AT 9000 FT. THIS WAS NO FACTOR TO THE FLT SINCE WE DID RE-ESTABLISH 2-WAY COM BTWN US. WE RECEIVED A DSCNT TO 4000 FT WHERE WE LEVELED OFF AND WERE HANDED OFF TO DULLES APCH FROM ZDC. DULLES APCH THEN CONFIRMED 4000 FT AND TO DEPART BARIN INTXN ON 320 DEG HDG TO INTERCEPT THE RWY 1L LOC WHEN WE CHKED IN. AFTER DEPARTING BARIN INTXN, ON A 320 DEG HDG DULLES APCH ASKED US OUR ASSIGNED ALT. I REPLIED 4000 FT. HE SAID HE WAS SHOWING US AT 3500 FT AND ASKED US OUR ALT. I TOLD HIM WE WERE LEVEL AT 4000 FT. THAT'S ALL THAT WAS SAID. THE CAPT AT THAT TIME SWITCHED TO THE SECOND XPONDER AND ASKED ATC WHAT WE WERE SHOWING ON HIS RADAR. ATC REPLIED 4000 FT.
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.