Reporter states mvy VOR signal inaccurate.

Attributes
ACN	108023
Time
Date	198903
Day	Fri
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : hya
State Reference	MA
Altitude	msl bound lower : 540 msl bound upper : 540
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tower : hya artcc : zob
Operator	common carrier : air carrier
Make Model Name	Small Transport
Navigation In Use	other vortac
Flight Phase	descent : approach
Route In Use	approach : straight in
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 250 flight time total : 3300
ASRS Report	108023
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	other anomaly other other spatial deviation
Independent Detector	other controllera
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Navigational Facility
Air Traffic Incident	Pilot Deviation other
Situations
Navigational Aid	Unspecified

Narrative:

We were shooting the back course mvy approach in WX reported as 400' overcast and 1 1/2 mi in light rain and fog. While inside the FAF (16.0 DME, mvy VOR) we noticed deviations of 1/2 to 1 degree. We corrected for what we thought were winds. Approximately 3 mi from the map (21.0 DME) tower advised us that we were 1/2 mi north of course. Our instruments showed that we were on course. At the map or just before reaching it we spotted the runway to our right by 1/4 mi. We turned right, lined up and landed safely. Later in the day we flew this approach twice and found the VOR course to be drifting at some points on the approach by as much as 1 1/2 dots in relatively short time periods. Callback conversation with reporter revealed the following: commuter airline. Reporter very familiar with this particular approach. Stated sometimes ships in the channel cause the VOR to swing. Allowable error on VOR test (before flight) xchking the 2 VOR instruments is 3 degrees. Believe 1 degree error at 21 mi would give approximately 1 mi off course. This is entirely possible on a back course at 21 mi from VOR.

Original NASA ASRS Text

Title: REPORTER STATES MVY VOR SIGNAL INACCURATE.

Narrative: WE WERE SHOOTING THE BACK COURSE MVY APCH IN WX RPTED AS 400' OVCST AND 1 1/2 MI IN LIGHT RAIN AND FOG. WHILE INSIDE THE FAF (16.0 DME, MVY VOR) WE NOTICED DEVIATIONS OF 1/2 TO 1 DEG. WE CORRECTED FOR WHAT WE THOUGHT WERE WINDS. APPROX 3 MI FROM THE MAP (21.0 DME) TWR ADVISED US THAT WE WERE 1/2 MI N OF COURSE. OUR INSTRUMENTS SHOWED THAT WE WERE ON COURSE. AT THE MAP OR JUST BEFORE REACHING IT WE SPOTTED THE RWY TO OUR RIGHT BY 1/4 MI. WE TURNED RIGHT, LINED UP AND LANDED SAFELY. LATER IN THE DAY WE FLEW THIS APCH TWICE AND FOUND THE VOR COURSE TO BE DRIFTING AT SOME POINTS ON THE APCH BY AS MUCH AS 1 1/2 DOTS IN RELATIVELY SHORT TIME PERIODS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: COMMUTER AIRLINE. RPTR VERY FAMILIAR WITH THIS PARTICULAR APCH. STATED SOMETIMES SHIPS IN THE CHANNEL CAUSE THE VOR TO SWING. ALLOWABLE ERROR ON VOR TEST (BEFORE FLT) XCHKING THE 2 VOR INSTRUMENTS IS 3 DEGS. BELIEVE 1 DEG ERROR AT 21 MI WOULD GIVE APPROX 1 MI OFF COURSE. THIS IS ENTIRELY POSSIBLE ON A BACK COURSE AT 21 MI FROM VOR.

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.