A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2017 Dec 25, 13:41 -0800
re : DavidPike-dec-2017-g41024
First of all, many thanks for your early reply, and for your very kind words about my upcoming long haul aviation retirement. I am impressed to communicate with a former Vulcan Navigator.
We fully agree on many points.
Yes ! The "heights averager" is an excellent system to get rid of short period oscillations : phugoid, dutch roll, turbulence induced auto-Pilot corrections among others. I agree that a "two minute averaging" somewhat represents a good trade off between pilots' ability to fly constant heading and some minimum elapsed time to zeroize the mean values of all such short periodic oscillations, with the Dutch roll or the phugoid probably being the longest period ones (probably around 40 seconds on an Airbus 340 but almost entirely damped thanks to a system which is still certainly one of the best ones on the market today).
About all of this we totally agree on.
Flying exact and very accurate Great Circle Tracks was outside available technology at earlier times, and for this reason either pilots or autopilots had to fly rhumb lines, i.e. constant heading curves, which actually were not even constant "true" heading curves since magnetic variation could significantly change over a quarter of an hour in flight.
When flying from point to point to-day which is the rule over the oceans Boeing or Airbus auto pilots no longer fly constant heading tracks - except when forced into HDG mode - but they rather fly Great Circles implying continuous very small heading changes. Only exception : North-South tracks anywhere and East-West tracks alongside the Equator. Great circles are nowadys followed extremely accurately. The "horizontal wandering" is in the order of just a few meters, with so many "exactly on top" encounters between aircraft in the middle of the oceans, often confirmed by radio altimeter alarms to the higher aircraft. This happens even when we use "magnetic heading" because the auto-pilots follow exactly the same great circle between any 2 given points. Magnetic or True Heading display has become of by-product of very performing Great Circle flying and not conversely. As result to-day auto-pilots coupled to both Inertial Navigation Systems and quite accurate (+/- 10 m) Satellite Navigation, it has become possible to fly excellent great circles. So, i still think that rhumb line tracking corrections should simply be ignored for the currently universal cases of Great Circle tracking.
However, one very important item was still missing and you named it : thank you very very much then for mentionning the Coriolis Acceleration. For interested readers, it is an acceleration due to the Earth Rotation and it is always perpendicular in space to the speed vector. It is absolutely insignificant for most naval applications, but it has quite sizeable effects on Aircraft given their much higher speeds.
I have come to the conclusion that I had wrongly ignored the Coriolis Acceleration in my earlier post.
Hence, as a final result, if the corrections linked to following a rhumb line are simply to be dropped as regards modern Airliners following "perfect" Great Circles, the Coriolis lateral displacement effects on apparent vertical always need to be accounted for. Do you agree ?