Narrative:

We had begun our descent from altitude for landing at rvs. We had been assigned a discretionary descent from FL330 to FL280. The altitude alerter/preselect was set to the assigned FL280. The autoplt was engaged and vertical speed mode was selected for the descent. This means that the autoplt is set to capture the preselected altitude. The aircraft was scheduled to descend at approximately 1500 FPM. Into the descent; ATC called us and asked our altitude. We had gone down to approximately FL243. I immediately checked the altitude alerter and saw that it was flagged. No audible had sounded and no light had illuminated to identify this failure. I immediately disconnected the autoplt to correct. We advised ATC that we had descended below our assigned altitude; were correcting; and notified them of our equipment failure. The controller acknowledged our report. Prior to handoff to local approach control; we were given a phone number and asked to contact ZKC after landing. After landing; I wrote up the broken altitude alerter/preselect unit for having failed. It remained flagged the entire way in on our arrival. I then called ZKC and reviewed the event. The phase of flight in which this event occurred is workload intensive. We had been preparing for our arrival which included getting the arrival ATIS; reviewing our landing data; programming the FMS; and briefing the type of arrival and destination FBO on the field. I allowed this workload to distract me from watching the altitude capture. The absence of an audible warning or annunciator light at the failure of this unit did not help. All failures/malfunctions in this cockpit have some type of warning light or horn. The fact that the autoplt stayed engaged in a descent path without warning of any abnormality in the altitude capture mode further aggravated this event. Lesson learned: never take your eyes off of the flight instruments and include the altitude preselect unit in your scan. I should have performed/briefed the other arrival duties after leveloff. Simply because a system starts out working correctly does not mean that it will not fail at the most unexpected time or in an unexpected manner.

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Original NASA ASRS Text

Title: B350 FLT CREW HAS ALTDEV WHEN ALT ALERTER FAILS DURING DSCNT.

Narrative: WE HAD BEGUN OUR DSCNT FROM ALT FOR LNDG AT RVS. WE HAD BEEN ASSIGNED A DISCRETIONARY DSCNT FROM FL330 TO FL280. THE ALT ALERTER/PRESELECT WAS SET TO THE ASSIGNED FL280. THE AUTOPLT WAS ENGAGED AND VERT SPD MODE WAS SELECTED FOR THE DSCNT. THIS MEANS THAT THE AUTOPLT IS SET TO CAPTURE THE PRESELECTED ALT. THE ACFT WAS SCHEDULED TO DSND AT APPROX 1500 FPM. INTO THE DSCNT; ATC CALLED US AND ASKED OUR ALT. WE HAD GONE DOWN TO APPROX FL243. I IMMEDIATELY CHKED THE ALT ALERTER AND SAW THAT IT WAS FLAGGED. NO AUDIBLE HAD SOUNDED AND NO LIGHT HAD ILLUMINATED TO IDENT THIS FAILURE. I IMMEDIATELY DISCONNECTED THE AUTOPLT TO CORRECT. WE ADVISED ATC THAT WE HAD DSNDED BELOW OUR ASSIGNED ALT; WERE CORRECTING; AND NOTIFIED THEM OF OUR EQUIP FAILURE. THE CTLR ACKNOWLEDGED OUR RPT. PRIOR TO HDOF TO LCL APCH CTL; WE WERE GIVEN A PHONE NUMBER AND ASKED TO CONTACT ZKC AFTER LNDG. AFTER LNDG; I WROTE UP THE BROKEN ALT ALERTER/PRESELECT UNIT FOR HAVING FAILED. IT REMAINED FLAGGED THE ENTIRE WAY IN ON OUR ARR. I THEN CALLED ZKC AND REVIEWED THE EVENT. THE PHASE OF FLT IN WHICH THIS EVENT OCCURRED IS WORKLOAD INTENSIVE. WE HAD BEEN PREPARING FOR OUR ARR WHICH INCLUDED GETTING THE ARR ATIS; REVIEWING OUR LNDG DATA; PROGRAMMING THE FMS; AND BRIEFING THE TYPE OF ARR AND DEST FBO ON THE FIELD. I ALLOWED THIS WORKLOAD TO DISTRACT ME FROM WATCHING THE ALT CAPTURE. THE ABSENCE OF AN AUDIBLE WARNING OR ANNUNCIATOR LIGHT AT THE FAILURE OF THIS UNIT DID NOT HELP. ALL FAILURES/MALFUNCTIONS IN THIS COCKPIT HAVE SOME TYPE OF WARNING LIGHT OR HORN. THE FACT THAT THE AUTOPLT STAYED ENGAGED IN A DSCNT PATH WITHOUT WARNING OF ANY ABNORMALITY IN THE ALT CAPTURE MODE FURTHER AGGRAVATED THIS EVENT. LESSON LEARNED: NEVER TAKE YOUR EYES OFF OF THE FLT INSTS AND INCLUDE THE ALT PRESELECT UNIT IN YOUR SCAN. I SHOULD HAVE PERFORMED/BRIEFED THE OTHER ARR DUTIES AFTER LEVELOFF. SIMPLY BECAUSE A SYS STARTS OUT WORKING CORRECTLY DOES NOT MEAN THAT IT WILL NOT FAIL AT THE MOST UNEXPECTED TIME OR IN AN UNEXPECTED MANNER.

Data retrieved from NASA's ASRS site as of January 2009 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.