Descent below assigned altitude by air carrier. Pilot deviation. Potential conflict with mlt.

Attributes
ACN	187532
Time
Date	199108
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : pmd
State Reference	CA
Altitude	msl bound lower : 11000 msl bound upper : 11500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : edw
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Operator	other
Make Model Name	Military Transport
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
ASRS Report	187532
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : crossing restriction not met altitude deviation : overshoot conflict : airborne less severe non adherence : clearance
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : reviewed incident with flight crew
Miss Distance	horizontal : 30000 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While working an air carrier on an en route descent to burbank, I instructed the aircraft to cross palmdale VOR at 12000 cross janny intersection at 8000 ft. The pilot read back the crossing restrictions verbatim. The pmd crossing restriction of 12000 was issued to separate from an opposite direction mlt at 11000 ft. When the 2 aircraft were 5 mi apart the air carrier was observed descending below 11000 ft even though it had not reached pmd VOR yet. Traffic was issued and the 2 aircraft passed without incident. In a conversation with the pilot of the air carrier, he advised that while he was reading back the crossing restrictions, he was also looking at lynxx 6 arrival into burbank. The lynxx 6 arrival chart advises 'planning information.' expect to cross pmd at or below 13000 ft. Although that is the instruction normally given, it was not in this instance. The pilot assumed the pmd crossing restriction to be 'at or below 12000 ft'. Lesson learned: cross 'at' means at the altitude, not above or below, regardless of the publications. If you have any questions, ask do not assume. Do not assume you will get the same instructions as always from air traffic because the traffic situation is never 'the same as before.'

Original NASA ASRS Text

Title: DSCNT BELOW ASSIGNED ALT BY ACR. PLTDEV. POTENTIAL CONFLICT WITH MLT.

Narrative: WHILE WORKING AN AIR CARRIER ON AN ENRTE DSCNT TO BURBANK, I INSTRUCTED THE ACFT TO CROSS PALMDALE VOR AT 12000 CROSS JANNY INTXN AT 8000 FT. THE PLT READ BACK THE XING RESTRICTIONS VERBATIM. THE PMD XING RESTRICTION OF 12000 WAS ISSUED TO SEPARATE FROM AN OPPOSITE DIRECTION MLT AT 11000 FT. WHEN THE 2 ACFT WERE 5 MI APART THE AIR CARRIER WAS OBSERVED DSNDING BELOW 11000 FT EVEN THOUGH IT HAD NOT REACHED PMD VOR YET. TFC WAS ISSUED AND THE 2 ACFT PASSED WITHOUT INCIDENT. IN A CONVERSATION WITH THE PLT OF THE AIR CARRIER, HE ADVISED THAT WHILE HE WAS READING BACK THE XING RESTRICTIONS, HE WAS ALSO LOOKING AT LYNXX 6 ARR INTO BURBANK. THE LYNXX 6 ARR CHART ADVISES 'PLANNING INFO.' EXPECT TO CROSS PMD AT OR BELOW 13000 FT. ALTHOUGH THAT IS THE INSTRUCTION NORMALLY GIVEN, IT WAS NOT IN THIS INSTANCE. THE PLT ASSUMED THE PMD XING RESTRICTION TO BE 'AT OR BELOW 12000 FT'. LESSON LEARNED: CROSS 'AT' MEANS AT THE ALT, NOT ABOVE OR BELOW, REGARDLESS OF THE PUBLICATIONS. IF YOU HAVE ANY QUESTIONS, ASK DO NOT ASSUME. DO NOT ASSUME YOU WILL GET THE SAME INSTRUCTIONS AS ALWAYS FROM AIR TFC BECAUSE THE TFC SITUATION IS NEVER 'THE SAME AS BEFORE.'

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.