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VOR/DME Arc Approaches for Pilots
by Neville van Eerten
This tutorial is to assist pilots in the correct way to make a VOR/DME approach via an arc into an attended aerodrome in Instrument Meteorological Conditions (IMC). The prerequisite is to have flown the
NDB approach tutorial into Ashburton.
In this tutorial we are not going to fly an approach between the radials of 120° to 240°, as that is similar to the NDB Approach tutorial with a hold and procedure turn. The base turn is not timed from the beacon, rather it is made at a specific DME distance from the Nelson VOR (10 miles).
Contents
In this tutorial we will make an approach into the Nelson aerodrome.
The surrounding area contains mountainous terrain to the east.
We will enter the L hand 15 DME arc from Blenheim (087° radial). Once established in the arc we will follow the arc via the steps to capture the 180° radial inbound for the VOR, then either commence a Circle to Land or a standard missed approach.
We are flying a ZK-PFT which is a light twin and is a Baron in FS2002. The aerodrome is controlled by Nelson Tower.
Not for real-life navigation purposes
The objective of the DME arc is to fly a fixed distance around a beacon. This means we can approach a beacon from almost any direction fly around it and intercept the final approach track. This means we do not have to fly all the way to the beacon, possibly have to fly a reversal turn, then go outbound on the approach, complete the base turn, and finally track inbound on the approach as we did in the NDB tutorial.
We need to remember one golden rule to follow this example: "needle centered with a 'to' flag". This means that before we start flying our arc we want the head of the CDI needle pointing at the beacon, the needle centered (on track), and the "to/from" flag pointing towards the beacon with a "to" flag.
I know these HSIs are not set for Nelson, but they were the only ones I could find. The DME should now indicate around 18°, so be prepared to turn very soon.
In our example we are tracking inbound to Nelson (NS) from Blenheim (BM) on the NS 087° radial (267° track). So on our HSI (Nav 1) we will have the course indicating arrow needle pointing at 267° and the CDI will be centered as it is in the example on the R (Note 1). Assuming we are on track, the heading bug will also be on 267° or close to it if the autopilot is flying the aircraft in heading mode and allowing for wind. We will be disconnecting the autopilot to fly this example, however it is possible to get the autopilot to make all the turns using the heading bug.
(Note 1: In the pictorial example the pilot is adjusting for L drift to keep the CDI in line and the plane on the 300° radial.)
PFT: Nelson Tower, Papa Foxtrot Tango is 18 miles from NS on the BM track at one zero thousand (10,000ft), with information November 1017 POB 5, top of descent, requesting the VOR/DME Alpha.
You have told Nelson that you are currently at 10000 feet 18 miles east, wanting to descend (top of Descent – TOD) and join the L hand arc for the approach (VOR/DME Alpha) shown above. You have the ATIS November QNH 1017 and you know from that, that the active runway is 02, which will require a circling approach to land.
ATC: Papa Foxtrot Tango, November 1017 is confirmed, descend to 8000, cleared VOR/DME Alpha runway 02, join DME Arc.
The controller has told you to descend to 8000 feet which is the minimum altitude for joining between radials 090° and 070°, and has cleared you all the way via the published DME arc.
PFT: Descend to 8000, cleared VOR/DME Alpha, for runway 02 via the Arc, Papa Foxtrot Tango
Time to slow this aircraft down because things are going to get REAL busy soon. Keep the RPM at 2400 (cruise) but drop the Manifold Pressure from 24 inches to 20 inches.
Check your ABRIEFS:
A = ATIS - you already have that: November 1017 (QNH is 1017)
B = Brief - the approach plate
R = Radios – keep NAV 1 and 2 on NS 116.40 – check Morse code ID
I = Instruments
E = Engines – temperatures and pressures etc – mixtures rich for the descent
F = Fuels – sufficient
S = security, time to buckle up the seatbelts
This is the Brief that you say out loud:
"VOR/DME Alpha approach via the L hand arc Nelson, Elevation 17 feet, enter the 15 DME arc at 8000 or higher (what the line below 8000 means), R turn into L hand arc (Note 2), crossing 070° radial descend to not below 6500, crossing 035° radial descend to not below 3500, L hand inbound course 180, Cat A Circle to land MDA is 660 feet (643 feet AGL) 2100m visibility required to commence the approach, missed approach point 1.5 DME climbing R turn, intercept track 315, turn right and enter the R hand arc and enter Tasman hold at 4000 feet or as instructed.
(Note 2: A new IFR pilot turned the wrong way into the arc at Napier and plastered himself into the mountain side by this simple mistake.)
What a mouthful, but exactly what you need to do – a very good reason to do it earlier. It will implant it into your mind. Go through the plate and see what everything means until you can do it by heart yourself.
It is generally expected that you can hold a VOR course within 2.5 dots of an intended radial.
Making the R turn to join the L arc.
Before we go any further I would like you to make sure you have a 3D picture of where we are on the NS VOR/DME chart. We were coming in from the right hand side of the chart, and just before 15nm we turned right towards the top of the chart.
Now where is the NS VOR in relation to us? It is on our left hand side, slightly behind the left wing tip ( I hope).
Remember at the start we said the golden rule was needle centered with a "to" flag. Looking at our HSI we find the needle is no longer centered. The needle is down at the bottom of the gauge so now we need to get it centered. The easiest way to center the needle is to point the head of the needle right at the beacon. Turn the course knob until the head of the needle moves back past the wing tip and the needle becomes centered. Once that is done we will move the needle an additional 10 degrees so now the CDI needle has gone full deflection towards the top of the gauge. While we are at it put the heading bug on 180° as a reminder of our inbound track. You will need the autopilot disconnected to do this.
Now look at the DME readout and I bet it is no longer saying 15.0nm.
What we are looking for is the little aircraft in the center of the HSI, 90 degrees to the CDI needle which is in the top half of the HSI and the DME saying 15.0
In our example you will find that the little aircraft is not at 90 degrees to our CDI needle, it is facing more towards the tail of the needle.
If we find the DME is less than 15nm, then having our little aircraft facing away from the head of the needle is fine. We want to fly out and away from the beacon until we get to 15 and then we will turn to the left so our little aircraft is at 90 degrees to the CDI needle and fly a constant 15 miles. You need to stay between 14 and 16 DME and preferably between 14.7 and 15.3DME. If our DME reads greater than 15 then we need to be turning left so our little aircraft is at 90 degrees or less and we are flying slightly towards the beacon.
While all this is going on the CDI needle will be moving steadily towards the center of the HSI and when it is centered we will turn the course knob a further 10 degrees so the needle goes back to the top of the gauge and we turn to fly 90 degrees relative to the needle (in no wind).
PFT: Papa Foxtrot Tango established on the arc.
ATC: Papa Foxtrot Tango
Since we have been cleared for the approach once we are established on the DME arc, we can descend according to the altitudes shown, remembering that you cannot descend until AFTER the 070° or the 035° radial then you will join the 360° radial (180° track) inbound for your approach.
We keep taking 10 degree bites at our arc until the head of the CDI needle meets our heading bug at 180°, which reminds us of our inbound track. We now turn left to intercept the track and fly inbound on the 180° track (360° radial) with the CDI needle centered down the final approach.
Now you can see why Civil Aviation requires a minimum of 40 hours flight training towards an instrument rating.
On the chart there are steps where you can descend to a lower altitude after passing a certain radial but how do I find that radial?
If you look at the tail on the CDI needle it is pointing at a number. When the needle is centered on this number this is the radial you are crossing. If the CDI needle is towards the top of the gauge then you haven't reached the radial selected. If the needle is towards the bottom of the HSI then you have passed that radial.
As we were taking our 10-degree bites we could have dialed up 250° on the CDI (070° radial) and when the needle centered we would be crossing this radial at which point we could descend to 6500 feet.
We could have also used Nav 2 to check these radials if you wanted. The same rule applies "needle centered with a to flag"
Slow to 120 knots by reducing the Manifold Pressure to 15 - 18 inches. Always keep above the blue line on your ASI until you are flaring for the landing.
Holds on the Arc
Holds on the arc will be covered in a separate hold tutorial under the subtitle, "Holds on the Arc", by Neville van Eerten.
When the CDI is 2 1/2 dots from the center on the 180° course you can turn L to make the final approach.
PFT: Tower, Papa Foxtrot Tango established inbound.
ATC: Papa Foxtrot Tango continue approach number one
Start the descent immediately because you are allowed to descend down to 3600 feet once established, and you need to attempt to get down to 2900 feet by 10 DME. You are number one for the approach. If there was traffic departing off of 02 (ie head on), then the tower may have made you hold at Tasman.
PFT: Papa Foxtrot Tango.
Time to follow the Profile for the approach:
Now is the time to make sure all checklists have been completed so we can concentrate on flying the approach.
At 10.9 miles you should say out loud to yourself: "10 miles 2900 feet." After passing ten miles say: "9 miles 2600 feet." After passing each mile you should say out loud the next altitude for the next DME distance. This will help you to get the right profile of descent. So you don't have to refer to the chart each time, this could be flown as a "3 times minus 100" profile. That is 3 times your DME distance minus 100 feet, e.g. at 4 miles we would have 4 x 3 = 12, minus 1 = 11 so 1100 feet at 4 miles.
Time to slow the plane down and do your downwind checks. (Shortened a bit)
B = Brakes - nothing more embarrassing than a slide into a fence
U = Undercarriage – three green lights
M = Mixtures rich – full rich
F = Fuel - fuel pumps on, landing lights on
For the rate of descent: Divide the airspeed by 2 then multiply it by 10 to get the feet per minute rate of descent for your Vertical Speed Indicator (VSI). You should currently be at around 120 knots so your rate of descent should be 120 divided by 2 = 60 x 10 = 600 fpm. Trim accordingly and you will find everything goes a lot easier.
Notice also at the bottom right is a diagram making it very clear that you cannot circle to the East. That is why the ATC has not told you to break right or break left, you're expected to know that.
Ok you are nearing MDA of 660 ft (643 AGL) and you are visual, so once you reach the Missed Approach point you can break right for a circle to land.
PFT: Papa Foxtrot Tango Visual, joining downwind 02
ATC: Papa Foxtrot Tango
Keep looking outside remembering we may only be 600 feet above the ground so now is not the time to have your eyes inside completing checklists. That is why we did the checklists earlier
PFT: Papa Foxtrot Tango downwind 02
ATC: Papa Foxtrot Tango, cleared to land 02
PFT: cleared to land 02, Papa Foxtrot Tango
Once on finals get these CUP checks done:
C = Cowl Flaps – open (if applicable to your aircraft)
U = Undercarriage - three green lights
P = Pitch – full fine, control your speed with your manifold pressure and extending more flaps (unless a strong cross-wind). Remember not to go below the blue line until you are able to land with an engine failure – just before or on the threshold
100 ft:
W = Wheels yes, the third time I checked the wheels for three green lights (Note 3)
W = Windsock.
(Note 3: A certain NZ pilot (no not me) was convinced that there were three green lights showing when he did a wheels up landing, and wrote that in his report. As no fault was found, the indications are that a mental picture was made in the mind of the pilot. He expected to see three green lights, and his brain therefore told him there were three green lights. Alway check - three green lights three times!)
Lets take a step back to approaching the MDA with a situation where you are not visual.
MISSED APPROACH: Climbing RIGHT turn, intercept R315, turn RIGHT and join the 15 DME arc, enter TASMAN holding 4000 or as instructed
You are at 1.5 DME inbound for the approach at 660 feet or just above and you cannot see the runway, or the visibility is less than 2100 meters.
Full power (Note 4), climbing turn to the R, set the heading bug to 345° (Note 5), climbing turn to the R, gear up, flaps up one notch at a time.
(Note 4: Don’t plaster the power, you may damage the engine, generally two to four seconds to increase power is ample.)
(Note 5: Generally you would bug then turn but in this case you would probably start your turn straight away, then bug once you have your climb established, but before you set the engines. The important thing about a missed approach is not to get panicky – it is a standard thing that happens everyday at all major airports. You need to move quickly, (don’t let that check pilot beside you think you are stressed!) but not hurriedly getting your checks done – prioritize: Aviate, Navigate, Communicate.)
PFT: Papa Foxtrot Tango commencing missed approach,
Flaps up one notch at a time
ATC: Papa Foxtrot Tango, hold at Tasman 4000 feet and advise intentions when ready
Power to 25 inches, RPM to 2500 and trim for the climb. Set the course-indicating arrow to 315°. Once you are safely away from the ground on the 315° radial, complete the after takeoff checklist.
PFT: hold at Tasman 4000 feet, wilco, Papa Foxtrot Tango.
Once you are established on the 315° radial you need to calculate when you will turn onto the radial, remember how to do it? The direction is easy, as you are flying away from the VOR your CDI is already centered. Just before you turn (when? (Note 6)) turn your heading bug to 90 degrees to the 315° radial and join the R hand arc. The hold will be covered in the hold tutorial for pilots.
(Note 6: Lets say you’re still climbing at 110 knots, so you start your turn 1.1 mile beforehand at 13.9 DME NS VOR.)
Congratulations on passing this difficult approach off the arc.
Neville van Eerten
neville@vetcall.co.nz
