ATPL blues. Gen Nav Theory Q.

[bultaco]

Hi Guys,

I'm suffering a severe case of the ATPL blues :(
Can someone please help with the following.....

An INS is loaded with 3 waypoints:

1) 64N 30W
2) 64N 20W
3) 64N 10W

Now here it goes...does the aircraft actually pass directly over the middle waypoint (fly over that geographic position 64N 20W) or does it do a great circle route from 1 to 3 ?

You would think that would be in the ATPL documents, however, the documents I have are far more interested in Ring Laser Gyros.

Bultaco

[Charlie Zulu]

If an INS is loaded with 3 waypoints then assume it will be flying between each of the waypoints on a great circle track.

Ie, it will fly between "A" and "B" on a GC and then alter its heading (if necessary) to fly from "B" to "C".

Assume all waypoints are flown over unless otherwise stated...

This type of question normally asks for the direction and number of degrees the aircraft will turn when it reaches "B", thus it'll usually be a nice simple convergency question.

If this is the case for this one then it'll be:

chLong * sin mean lat.

So, 10 * sin 64 = 8.9879, which is 9 degrees for the purpose of this question.

As the GC heading at "B" will be greater than the RL at "B" and the RL from "B" to "C" is the same as "A" to "B" (090), then the aircraft heading will have to turn left 9 degrees (or 8.9 is a bit more accurate).

[Alex Whittingham]

If the INS is in AUTO it will go from 1 to 2 then from 2 to 3.
If you select 'Waypoint Change' '1' '3' 'Enter' it will take up a track from 1 to 3 completely missing out 2.
If you select 'Waypoint Change' '0' '3' 'Enter' it will go direct from present position to waypoint 3.

In the first case, which is what you are probably talking about, it doesn't actually go right over the top of waypoint 2 unless it is on track. If there's a turn it cuts the corner and passes 'close to' the waypoint rather than overflying it, then shooting out the other side and having to make a large correction back to track.

In the case of the positions you have given the turn is so small that there's not much of a corner to cut and it will pass very close indeed to waypoint 2, certainly within 400 yards.

Its an odd question. Is it a progress test?

[bultaco]

Thanks Charlie Zulu for the quick reply. I take my hat off to anybody who has done these exams. It's not like studying something very clear-cut...Like there's many conflicting ATPL feedback questions which send you one way and then another on a great circle route :)

Bultaco

[bultaco]

Hi Alex,

Thanks for your reply, yes it's a progress test question which also involved finding the track change on passing waypoint 2. Charlie Zulu has calculated the track change and it's the correct answer according to the answer key...great :)

Bultaco

[bultaco]

Yeah, the ATPL syllabus should not assume that one knows about something that's not in a Cessna 150.