ALT XING RESTR NOT MET. ALTDEV ALT UNDERSHOT. DME ERRATIC.

Narrative

WHILE PROCEEDING DIRECTLY TO THE PALMDALE VOR AT 78 DME E AND AT FL280; WE RECEIVED CLRNC TO CROSS 10 E OF THE PALMDALE VOR AT 14000 FT. I COMPUTED A DSCNT POINT OF 60 NM E. AT 60.2 DME E OF THE PALMDALE VOR; I STARTED THE DSCNT AND WE CALLED OUT OF FL280. I WAS DSNDING AT 2400 FPM AND PICKED UP 310 KTS WITH THE THROTTLES AT IDLE. AT 40 DME OF THE PMD VOR; WE WERE PASSING FL235 WHICH WAS A SATISFACTORY CHK OF OUR DSCNT RATE. APPROX 1-2 MINS LATER THE CTLR TOLD US WE WERE 10 E OF PALMDALE AND NOT AT 14000 FT. AT THAT MOMENT; THE DME READ 9.6 AND WE WERE PASSING FL205. AS A CREW; WE WERE BEWILDERED BY THE LOSS OF 40 MI. I TOLD THE CTLR THAT WE HAD EXPERIENCED A 40 MI CHANGE IN DME. WE WERE GIVEN A DELAYING TURN TO MAKE SPACE FOR THE REST OF OUR DSCNT. AFTER LNDG THE FO AND I DISCUSSED THE POSSIBILITY THAT THE VAN NUYS VOR WAS SELECTED IN HIS NAV; BUT THAT WOULD HAVE GIVEN US A 30 DME DIFFERENCE AND NOT 40 AS WE HAD EXPERIENCED. WE ALSO DISCUSSED THE POSSIBILITY OF DME SIGNAL INTERFERENCE EITHER FROM WITHIN THE ACFT OR FROM EXTERNAL SOURCES. SUPPLEMENTAL INFO FROM ACN 270593: AN EXPLANATION FOR THE LOSS OF 40-50 MI IS DIFFICULT. THE #1 VOR WAS BEING USED FOR NAV AND WAS NEVER CHANGED THROUGHOUT THE EVENT. #2 VOR WAS UPDATED TO PMD DURING THE DSCNT.