Advanced technology air carrier medium large transport undershot crossing restriction on descent.

Attributes
ACN	88210
Time
Date	198805
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : cvi
State Reference	NC
Altitude	msl bound lower : 33000 msl bound upper : 34000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	common carrier : air carrier
Make Model Name	Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	descent other
Route In Use	enroute airway : zdc
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 240 flight time total : 13000
ASRS Report	88210
Person 2
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	altitude deviation : crossing restriction not met altitude deviation : undershoot non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

The aircraft was an large transport en route from mia to dca. Deviating around thunderstorms, using the offset feature of the LNAV. Given a clearance to cross 40 mi south of cvi at FL330. The fix was created and entered into the VNAV on a crossing restriction. The T/D point was created by the computer. When approximately 70 mi out, the offset was cancelled and the plane started back to the original course. When approximately 3 mi south of the 40 mi fix (according to the RNAV) the center called and asked why we were not level. The IRU or RNAV position still showed us with 2 1/2 mi to go. Then center insisted that we were 40 mi out of civ. The basic problem with this is that on the large transport, all dscnts and climbs are based upon IRS based position generation of the IRU's. In this case, the IRU's position showed 3.1 mi to go before the fix. Also, the VNAV was working the descent. Neither pilot had any position readout from the VOR at cvi because the VOR's were in automatic and the FMC's determined VOR tuning.

Original NASA ASRS Text

Title: ADVANCED TECHNOLOGY ACR MLG UNDERSHOT CROSSING RESTRICTION ON DESCENT.

Narrative: THE ACFT WAS AN LGT ENRTE FROM MIA TO DCA. DEVIATING AROUND TSTMS, USING THE OFFSET FEATURE OF THE LNAV. GIVEN A CLRNC TO CROSS 40 MI S OF CVI AT FL330. THE FIX WAS CREATED AND ENTERED INTO THE VNAV ON A XING RESTRICTION. THE T/D POINT WAS CREATED BY THE COMPUTER. WHEN APPROX 70 MI OUT, THE OFFSET WAS CANCELLED AND THE PLANE STARTED BACK TO THE ORIGINAL COURSE. WHEN APPROX 3 MI S OF THE 40 MI FIX (ACCORDING TO THE RNAV) THE CENTER CALLED AND ASKED WHY WE WERE NOT LEVEL. THE IRU OR RNAV POS STILL SHOWED US WITH 2 1/2 MI TO GO. THEN CENTER INSISTED THAT WE WERE 40 MI OUT OF CIV. THE BASIC PROB WITH THIS IS THAT ON THE LGT, ALL DSCNTS AND CLBS ARE BASED UPON IRS BASED POS GENERATION OF THE IRU'S. IN THIS CASE, THE IRU'S POS SHOWED 3.1 MI TO GO BEFORE THE FIX. ALSO, THE VNAV WAS WORKING THE DSCNT. NEITHER PLT HAD ANY POS READOUT FROM THE VOR AT CVI BECAUSE THE VOR'S WERE IN AUTO AND THE FMC'S DETERMINED VOR TUNING.

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.