Line of Latitude or Longitude??

[Duffer2007]

Came across this question while studying. Is it easier to fly a line of longitude or a line of latitude? If someone can explain their answer please. I have an idea but don't want to influence anyones thoughts.

Cheers

[mad_jock]

Which one usually has an instrument that points ruffly along it.

[G CEXO]

Latitude.

I'd love to see someone fly a longitude great circle without INS/IRS near the poles :p

[Darth_Bovine]

Sorry to dredge this one up, but I was asked this recently.

Surely if your flying a line of longitude then it is a great circle but is also constant heading (i.e. north or south).

If you're flying latitudes then you are also flying a constant heading (although only the equator is line of latitude which is also a great circle track).

So can anyone answer the original question: "Is it easier to fly a line of longitude or latitude?" and give reasons?

Thanks,
DB.

[mad_jock]

Longtitude or zero latitude.

Because if you keep a constant heading you will fly a rhumb line if you want to fly a great circle you will have to change the heading as you go along.

On a north south Longitude line the rhumb line is the same as the great circle.

[Alex Whittingham]

I started writing a long answer to this, but it got very long because the answer changes depending on the assumptions you make about the nav kit you are using, the location on the earth and the rate of change of variation and wind vector - and what the question means by 'easier'. I'm afraid that, without these qualifications, I can't give an answer.

[Darth_Bovine]

Hi Mad Jock,

Thanks for the reply.

Maybe I'm missing something but the original question (and my question) was: what's easiest to fly: a line of Latitude or a line of Longitude.

It didn't actually mention great circles although I agree that what you say is correct regarding those.

Maybe the answer is they are both equally easy to fly? If you fly constant Lat you will be flying a rhumb line (unless it's along the equator), but it's still constant heading so no trick there. Similarly for Longitude you will be flying a constant heading but by definition it will follow a great circle track.

It's been a while since I studied Gen Nav so I may be off the mark. I appreciate any guidance.

This sounds like a potential interview question - hence my interest.

Cheers,
R.

[Darth_Bovine]

Hi Alex,

Thanks for taking the time to think about this. Maybe there is too little info in the question and the interviewer just wants you to do some problem solving out loud...

Hope BGS is still busy!

-DB (BGS graduate!)

[merch]

Could it be Latitude is easier to fly?

If you flew a line of longitude you would have allow for the coriolis effect, the line (meridian) of longitude would "move" underneath you and you would be constantly chasing it.

[mad_jock]

If you are going to fly a constant latitude away from the equator you are going to have to get your calculator out and constantly change your heading or shall we say direction if using a gyro.

If your fly a Longitude you just set your gyro pointing in North or South direction and just keep pointing the same way.

the whole thing falls over though because of magnetic North pole so in reality its not as easy as you might think apart from a very limited area both sides of the world so realistically you have to forget about a compass so that leaves a gyro which has drift etc etc.

So I would class the question as theoretical bollocks which has no real application to the real world. And pointless asking because its a easy remember job from a question bank so a such doesn;t renforce the concept which it is very poorly trying to test.

[Darth_Bovine]

If you are going to fly a constant latitude away from the equator you are going to have to get your calculator out and constantly change your heading or shall we say direction if using a gyro.

Ignoring all drift winds effect etc... am I not correct in saying: if you are on a line of Lat (say 50N for example) and head due east then I will remain on that latitude? I.e. following a constant heading will keep me on that Lat?

I've confused myself! Looking at the convergency formula contradicts what I intuitively feel to be correct (i.e. what I stated above).
Convergency = change long x sine(mean Lat) = change of GC track.

Unless that only holds true for great circles in which case my assumption of heading east on a line of Lat (rhumb line) only requires a constant heading...

Help please!

[Alex Whittingham]

Yes, if you fly a constant heading of 090°T in still air you will follow a line of latitude (a rhumb line), but (i) the air is not still, as the wind changes you will have to change your heading (ii) very few light aircraft compass systems (none?) allow you to fly true heading so you will actually have to work out variation and fly magnetic heading. Variation changes so you will also have to change your mag heading to allow for that and (iii) both the above arguments apply equally to a true track of 000° which would allow you to fly up a meridian (both a great circle and a rhumb line).

If you are flying on a DI these are affected by transport wander, earth rotation and the possibly erroneous effect of the latitude nut. Flying due north or south transport wander would be zero, flying along the latitude the latitude nut was set to compensate for would remove that error.

If you are flying with an IRS/FMS combination coupled to the autopilot everything is *easier*. Flying due north or south you can just follow the great circle track, flying east or west you would have to build an approximation to the rhumb line with a series of short (50NM to 80NM) great circle tracks. This is marginally less *easy* but would still give much greater navigational accuracy than trying to stumble along a line of latitude in a light aircraft with a compass and whizz wheel.

Maybe this is just a discussion question with no correct answer, to see how much you know?

[Alex Whittingham]

Merch, you would be flying in the air mass and, while that will probably be affected by the coriolis effect (and other forces) and therefore move relative to the surface we see that as wind, and compensate accordingly.

[merch]

Thanks Alex, I sit corrected