Altitude excursion results in a potential conflict with less than standard separation.

Attributes
ACN	123717
Time
Date	198909
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : flo
State Reference	SC
Altitude	msl bound lower : 24500 msl bound upper : 25000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other descent other
Route In Use	enroute airway : zjx
Flight Plan	IFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : instrument pilot : cfi pilot : atp
Experience	flight time last 90 days : 78 flight time total : 8624 flight time type : 619
ASRS Report	123717
Person 2
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 63 flight time total : 9556 flight time type : 2036
ASRS Report	123767
Events
Anomaly	altitude deviation : excursion from assigned altitude altitude deviation : overshoot conflict : airborne less severe non adherence : clearance non adherence : required legal separation
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance flight crew : returned to intended course or assigned course other
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 10000 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

I set FL240 on the altitude alerter instead of FL250. At 24000' ZJX controller asked for out altitude, and I replied what I read on the altimeter. Simultaneously, I saw another aircraft crossing diagonally right to left, near our altitude, approximately 1 1/2-2 mi away. We were instructed to climb back to FL250, which we did. No evasive maneuvering was required. The captain called ZJX after landing and was told that I read back FL250. I do not know why this happened, except that we fly this route regularly and normally get descent clearance to FL240 at this approximately location, and perhaps I was expecting FL240.

Original NASA ASRS Text

Title: ALT EXCURSION RESULTS IN A POTENTIAL CONFLICT WITH LESS THAN STANDARD SEPARATION.

Narrative: I SET FL240 ON THE ALT ALERTER INSTEAD OF FL250. AT 24000' ZJX CTLR ASKED FOR OUT ALT, AND I REPLIED WHAT I READ ON THE ALTIMETER. SIMULTANEOUSLY, I SAW ANOTHER ACFT XING DIAGONALLY RIGHT TO LEFT, NEAR OUR ALT, APPROX 1 1/2-2 MI AWAY. WE WERE INSTRUCTED TO CLB BACK TO FL250, WHICH WE DID. NO EVASIVE MANEUVERING WAS REQUIRED. THE CAPT CALLED ZJX AFTER LNDG AND WAS TOLD THAT I READ BACK FL250. I DO NOT KNOW WHY THIS HAPPENED, EXCEPT THAT WE FLY THIS ROUTE REGULARLY AND NORMALLY GET DSCNT CLRNC TO FL240 AT THIS APPROX LOCATION, AND PERHAPS I WAS EXPECTING FL240.

Data retrieved from NASA's ASRS site as of August 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.