LR35 crew in foreign airspace believed they had received a clearance to FL350.

Attributes
ACN	493792
Time
Date	200012
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : mmsd.airport
State Reference	FO
Altitude	msl single value : 33400
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : mmex.artcc tower : atl.tower
Operator	common carrier : air taxi
Make Model Name	Learjet 35
Operating Under FAR Part	Part 135
Flight Phase	cruise : enroute altitude change
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : mmex.artcc
Operator	common carrier : air carrier
Make Model Name	B767 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 180 flight time total : 4000 flight time type : 1900
ASRS Report	493792
Person 2
Affiliation	company : air taxi
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Miss Distance	horizontal : 6000 vertical : 400
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Ambiguous

Narrative:

Departed mmsd. Instructed to climb to FL310. After climbing through FL250, instructed to climb to FL280. After leveling off at FL280, instructed to climb to FL350. After leveling at FL350, received radio call for our aircraft but was almost completely unreadable. Approximately 10 mins later, received another call that was readable. The controller said that we were not cleared to FL350 only FL310 and tht we came within 10 mi of a B767 at FL330 (10 mi separation in mexico's airspace). After each assignment, we read back our assignment twice and was confirmed. I called mazatlan center when I landed and they said that no report would be filed for numerous reasons -- first being the poor radio reception in the area, second, center radios calls to the B767 went unanswered. Contributing factors: 1) very poor radio reception. 2) B767 TCASII system may not give RA because of nonstandard 10 mi separation. Human performance considerations; 1) very strong spanish accent of center controller.

Original NASA ASRS Text

Title: LR35 CREW IN FOREIGN AIRSPACE BELIEVED THEY HAD RECEIVED A CLRNC TO FL350.

Narrative: DEPARTED MMSD. INSTRUCTED TO CLB TO FL310. AFTER CLBING THROUGH FL250, INSTRUCTED TO CLB TO FL280. AFTER LEVELING OFF AT FL280, INSTRUCTED TO CLB TO FL350. AFTER LEVELING AT FL350, RECEIVED RADIO CALL FOR OUR ACFT BUT WAS ALMOST COMPLETELY UNREADABLE. APPROX 10 MINS LATER, RECEIVED ANOTHER CALL THAT WAS READABLE. THE CTLR SAID THAT WE WERE NOT CLRED TO FL350 ONLY FL310 AND THT WE CAME WITHIN 10 MI OF A B767 AT FL330 (10 MI SEPARATION IN MEXICO'S AIRSPACE). AFTER EACH ASSIGNMENT, WE READ BACK OUR ASSIGNMENT TWICE AND WAS CONFIRMED. I CALLED MAZATLAN CTR WHEN I LANDED AND THEY SAID THAT NO RPT WOULD BE FILED FOR NUMEROUS REASONS -- FIRST BEING THE POOR RADIO RECEPTION IN THE AREA, SECOND, CTR RADIOS CALLS TO THE B767 WENT UNANSWERED. CONTRIBUTING FACTORS: 1) VERY POOR RADIO RECEPTION. 2) B767 TCASII SYS MAY NOT GIVE RA BECAUSE OF NONSTANDARD 10 MI SEPARATION. HUMAN PERFORMANCE CONSIDERATIONS; 1) VERY STRONG SPANISH ACCENT OF CTR CTLR.

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.