An small aircraft flying for instrument instruction assumes clearance to descend, and descends 500 ft below assigned altitude. ZME was controling at the time.

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Attributes
ACN	382270
Time
Date	199710
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : msl
State Reference	AL
Altitude	msl bound lower : 2500 msl bound upper : 3000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zme
Operator	general aviation : personal
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 91
Flight Phase	descent other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	instruction : instructor
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 100 flight time total : 7200 flight time type : 15
ASRS Report	382270
Person 2
Affiliation	Other
Function	instruction : trainee
Qualification	pilot : instrument pilot : private
Experience	flight time last 90 days : 30 flight time total : 720
ASRS Report	383080
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While acting as flight instructor, our aircraft was executing a practice ILS runway 29 approach to MSL, al, airport. ZME cleared aircraft to intercept localizer. We mistakenly assumed that we were cleared to the next stepdown altitude rather than maintain previously assigned 3000 ft. Controller questioned our mode C readout at which time we were approximately 500 ft below assigned altitude. We were then cleared for the approach. WX was clear with approximately 20 mi visibility. Center seemed to be very relaxed in their instructions regarding our training request. Specifically, given clearance to enter holding at oakla 'let us know when you get ready to do something else.' ie, no efc time. Also, 'cleared for VOR approach.' we executed the VOR 29 approach which was consistent with our earlier request. Center was under the impression we were executing the runway 11 approach. Supplemental information from acn 383080: we inadvertently felt we were cleared for approach and proceeded to next stepdown altitude rather than earlier altitude assigned of 3000 ft. ZME questioned our mode C display at which time we were at 2500 ft or 500 ft off altitude.

Original NASA ASRS Text

Title: AN SMA FLYING FOR INST INSTRUCTION ASSUMES CLRNC TO DSND, AND DSNDS 500 FT BELOW ASSIGNED ALT. ZME WAS CTLING AT THE TIME.

Narrative: WHILE ACTING AS FLT INSTRUCTOR, OUR ACFT WAS EXECUTING A PRACTICE ILS RWY 29 APCH TO MSL, AL, ARPT. ZME CLRED ACFT TO INTERCEPT LOC. WE MISTAKENLY ASSUMED THAT WE WERE CLRED TO THE NEXT STEPDOWN ALT RATHER THAN MAINTAIN PREVIOUSLY ASSIGNED 3000 FT. CTLR QUESTIONED OUR MODE C READOUT AT WHICH TIME WE WERE APPROX 500 FT BELOW ASSIGNED ALT. WE WERE THEN CLRED FOR THE APCH. WX WAS CLR WITH APPROX 20 MI VISIBILITY. CTR SEEMED TO BE VERY RELAXED IN THEIR INSTRUCTIONS REGARDING OUR TRAINING REQUEST. SPECIFICALLY, GIVEN CLRNC TO ENTER HOLDING AT OAKLA 'LET US KNOW WHEN YOU GET READY TO DO SOMETHING ELSE.' IE, NO EFC TIME. ALSO, 'CLRED FOR VOR APCH.' WE EXECUTED THE VOR 29 APCH WHICH WAS CONSISTENT WITH OUR EARLIER REQUEST. CTR WAS UNDER THE IMPRESSION WE WERE EXECUTING THE RWY 11 APCH. SUPPLEMENTAL INFO FROM ACN 383080: WE INADVERTENTLY FELT WE WERE CLRED FOR APCH AND PROCEEDED TO NEXT STEPDOWN ALT RATHER THAN EARLIER ALT ASSIGNED OF 3000 FT. ZME QUESTIONED OUR MODE C DISPLAY AT WHICH TIME WE WERE AT 2500 FT OR 500 FT OFF ALT.

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.