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Hi All, anyone who can help me to explain what might be wrong in the situation below. Currently, the pilot (chopper) is saying there is a 3-5degree deflection to the right (at 40nm) when flying to the rig. My question is, since NDB is non directional, how will there be a 3-5 degree deflection? What is causing this deflection? I am assuming the deflection is showing on the ADF onboard this is quite a constant error but not fixed. Sometime the pilot will say 5-10deg to the right. As i am not a certified pilot, i am not sure what is wrong. Hope one can assist to better explain.
dpk1nba's picture
Sounds to me like the ADF is
Sounds to me like the ADF is inoperative. The ADF needle always points to the NDB station, as long as it is getting a good signal from the station, no matter what direction you are flying. If the ADF needle is not pointed to the NDB station, and you are getting a verifyable signal from the station, the ADF is not working properly.


Could it be the pilot has
Could it be the pilot has both a GPS and NDB on board, and is tracking the GPS direct to station and noticing the ADF needle is deflecting 5-10 deg off the GPS heading to station. I see this on occassion when flying my plane, which has both GPS & NDB. Weak NDB stations are one possible cause, AM radio interference and marginal frequency discrimination in the NDB set are a couple others. The more obvious situation is when the AC heading is off course to compensate for wind, then the GPS track would be true while the NDB heading would be off by the heading vs course track difference.

GeorgeP CFII

NDB tracking

There will be a desired track to follow straight line from the NDB to the rig, say 350°M. Since the ADF needle points directly the NDB at all times, the head of the needle would be pointing behind to 070° if the pilot was on track. Given that the needle deflection was say 5° to the right, the tail of the needle would be reading 355°M (and the tail indicating 075°). In this case, the helicopter is 5° to the right of track. Using a rule known as the 1 in 60 rule, one can deduce that the helicopter is 3.33nm off track to the right.


I hope this helps.