For today’s lesson, we wanted to cover:
- Position fixing
- Radio navigation using the VOR
- Radio navigation using the ADF/NDB
Position fixing is one of the (many!) things that will be in the skills test, but is also an important thing to be able to do incase you become ‘temporarily uncertain of position’ (lost).
There are lots of ways to identify your position, including asking the radio service for a True Bearing (or QTE) or Magnetic Heading To Station (QDM). If you get a QTE, simply plot the line on the chart and you will be somewhere along that line. With a QDM, you first need to convert magnetic to true, then plot the reciprocal (as QDM is direction to station).
However today we would be looking at position fixing using radio navigation aids – the VOR, ADF/NDB and DME.
VOR is short for VHF omnidirectional range, and is technology that dates back to the 1940s but is still in wide use today. To use a VOR, you need a VOR receiver in your aircraft. You then need to be in range of a VOR station. There are many across the UK and they have a fairly long range so this shouldn’t be much of an issue. To use one:
- Tune – the nav radio to the VOR frequency
- Ident – listen to the morse to check you are receiving the right station, and that it is in service
- Twist – check the warning flag is out, and twist the OBS knob 360 and check you get the FROM and TO flags accordingly, then set the desired radial
- Sensible – does the indication seem sensible?
The direction flag is really important. If you wanted to fly along the 090 radial from a VOR, you’d make sure you had the FROM flag displaying. If the needle is to the left of centre, you turn left until the needle centres. If you want to fly TO a VOR, you’d make sure the TO flag is set.
So, we can use this to identify the radial we are over. Tune and ident, then twist until the flag is on FROM and the needle is centred. Lets say it is centred on 070. We are somewhere along the line of the 070 radio from the VOR. Most VORs also have a DME (distance measuring equipment) which gives us the distance from the VOR. We can then plot the distance along the line to get our exact fix. Alternatively, tune to any DME in range, get the distance and plot it on the chart, which should be somewhere along the line you’ve drawn.
We could also identify our position without a DME, but this would require to VORs. First, we’d plot the radial from VOR1 and then plot the radial from VOR2. Where the two radials cross, that is where we are.
On the other hand, the ADF (Automatic Direction Finder) is a simpler instrument that points towards the station (an NDB, non directional beacon). To use one, again use the TITS mnmemonic – tune, ident, test (there is usually a test button on the DME), sensible – is it pointing in the direction we expect?
To use an ADF for position fixing, make sure you’re flying straight and level. Then twist the heading dial on the ADF to match your current heading. The arrow on the ADF will be pointing to the heading you need to fly TO the NDB. To plot our current position, simply take the reciprocal (the tail of the needle) on the chart from the station. Again, with a DME we can then plot our exact position along that line.
I’ve covered Diversions before, but to recap – make sure you plan the diversion from a known point. Draw a line between the two points. Estimate (or ideally measure) the angle, convert from degrees true to magnetic, correct for wind drift, then fly the heading. Make a note of the time in your PLOG. Next, measure the distance and calculate how long it will take (allowing for wind). Update the PLOG with an ETA. Mark off the quarter/half/three quarter way points, and look for distinguishing features along the route. Keep looking out to make sure you are on track.
The flying went well – we flew out of Gloucester towards May Hill (West) then tuned to the Brecon VOR. I fixed our position using the VOR – we were on the 075 radial FROM brecon, at 32 miles. Then we flew along the 075 radial for a bit, by flying TO the 255 radial. After about 10 miles, we flew north. We wanted to intercept the 065 radial FROM the vor, so that we can track it towards Ross on Wye. Flying North, I tuned 065 FROM in the VOR and waited for the needle to center. Then I turned to a heading of 065 and kept it centered. After about 5 miles we were over Ross. Pretty cool.
I then had to divert from Ross to Bromyard, which required a heading of 010, and at 16 miles was estimated to take 10 minutes. I had the Ledbury aerial mast, and the towns of Hereford and Ledbury either side of me to confirm my position along the route, as well as the high ground, M50 motorway and a railway line. Sure enough, after 10 minutes we were right over Bromyard (or about 1 mile to the right of it, to be exact – but pretty darn close).
Diversion complete, we used the ADF to position fix and verify that we are actually over Bromyard. So I tuned to Gloucester’s NDB, ident and tested, then aligned the dial to our heading. The tail of the needle was pointing to 320, which I drew on the chart. Sure enough, that line goes straight through Bromyard. Our DME was out of service, so I couldn’t verify it exactly, but I was confident it was correct.
As if by magic, our engine caught fire (well, not really – the instructor pulled the throttle back to idle and told me to carry out a practice engine fire drill). First things first – fuel off, throttle closed, mixture ICO, fuel pump off, heater/demist off. Then carry on as if it was a normal PFL – trim for 73, identify a field, MAYDAY call, Squawk 7700, mags off, seat belts tight, door unlatched, master switch off, first stage of flaps when you are confident you’ll make it to 2/3rds along the landing point, second stage when you’re confident of reaching 1/3rd, then.. then we climbed away. All good except it took me a while to remember the initial steps. Must learn them some more.
We then put the screens up and I practiced instrument flight back to about 5nm from Gloucester, where I joined right base for runway 27.
Next lesson will be a dual nav exercise to Wolverhampton Halfpenny Green.