Hey guys

Just want to clarify ONUS in regards to the compass. Does it mean the compass over shoots when you turn onto north or are you supposed to over shoot your heading when you turn onto north. Same with undershooting south, does it mean the compass undershoots or are you actually supposed to undershoot the heading change ?

Kind Regards
Johnny

Hi Johnny,

The compass is Nippy on North (reads ahead of the turn) and Sluggish on South (reads behind the turn). That means:

- Because the compass is overreading the turn through North, you need to keep your turn going to a heading past the one you want, otherwise you will not turn far enough (you overshoot the reading you want on a Northerly turn).

- Because the compass is underreading the turn through South, you need to stop your turn on a heading earlier than the one you want, otherwise you will turn too far (you undershoot the reading you want on a Southerly turn).

To remember if ONUS applies to you or the compass, I usually tell students the "onus" is on them to "Overshoot North Undershoot South". One student told me he rememebered it by saying the pilot is the one with the "anus" (onus)

Cheers,

Rich

haha thanks Richard makes perfect sense now, its always good when you can try to remember something by having a laugh.

I had a practice question on this, turning from 000 to 340. Turning away from Nth I chose the “undershoot” answer. Apparently overshoot was correct. Please explain?

The phrase 'overshoot north' applies to any heading in the northern half of the azimuth scale. The amount to overshoot by is maximum on north itself, and varies to zero on east or west. You should interpolate the overshoot allowance for any heading between 000 and east or west. It doesn't matter what direction (left or right) you are approaching the target heading from.

The same applies to the undershoot allowance for southerly headings.