Gen Nav Question Help

[Phenom100]

Guys,

Please help me with the following question?

Given: Position NDB (55°10´N, 012°55´E) DR Position (54°53´N, 009°58´E) NDB on the RMI reads 090°. Magnetic variation = 10°W. The position line has to be plotted on a Lamberts conformal chart with standard parallels at 40°N and 48°N. Calculate the direction of the bearing to be plotted from the NDB.;

Many Thanks

Dan

[billa]

RMI reads in your aircraft to the NDB, so the bearing from the beacon to you is the reciprocal bearing. However, we have to look at variation and convergency.

Variation is applied at the point where the bearing is measured - in this case at the aircraft. We are told that it is 10°W. (If this were a VOR radial, then we'd apply at the VOR). Variation West, Magnetic Best. Therefore the true bearing to the NDB is going to be 10° less than 090° at 080°.

Now we want to plot our position as a bearing from the NDB. Radio waves travel in great circle lines, so we need to look at convergency. We are plotting on a lambert's chart where convergency = change in longitude * sin(mean standard parallel). So, the change in longitude between us and the NDB is 012°55' - 009°58' = 2°57' = 2.95°. The mean standard parallel is 44°N (40+48 / 2). So convergency = 2.95 * sin(44) = 2°.

Draw a diagram with 2 meridians sloping inwards towards true North (we are in the Northern hemisphere). A line running roughly West-East and slightly South-North gives us a rough indication of the bearing between aircraft on the left meridian and NDB on the right. We know that it's 080° True TO the NDB from the aircraft and can see that at the NDB this "track" has a greater true angle - the difference being convergency - so we add it on. 082° at the NDB. Except we want the bearing from the NDB to us, so need to add 180°. Answer: 262° True from the NDB to our position.

[Daniel777]

Hey there,

Another quick Gnav question about plotting:

If they ask "an aircraft is on the 025 radial from SHN VOR az 49 DME.What is its position?"

In this case do I align the protractor to Magnetic(arrows on the chart) or True North?(align with meridian)

Generally on the E(LO) 1 chart if they say radial is it Magnetic or True?

Thanks alot,

Daniel

[chrisbl]

If it is a radial from a VOR then it will be magnetic.

They question is asking for the lat & Long.

[bfisk]

Well, it won't be either, actually. It will normally be the magnetic, or what was magnetic when the VOR was first put into operation. The E(LO) charts will list VOR variations in a table. So you would have to correct the radial, using this information, into a true bearing, before plotting...