How to fly DME arcs?

[Pilot_Ryan]

It has nothing to do with TAS. The turn needs to be predicated off GROUNDSPEED.

No, it's TAS.

The airplane drifts with the wind during the turn. The groundspeed is not constant because the heading is constantly changing. Say for example that an airplane flying 120 KTAS has a direct tailwind of ten knots. As the standard rate turn begins to intercept the arc, the airplane drifts by windspeed/120, or about 0.083nm per minute. Round that up to 0.1nm and that's about how far you'll drift during the intercept - tack it on if you're so inclined (the rule of thumb doesn't include this because it's not really worth the trouble.)

In reality, this doesn't matter much. Use groundspeed or TAS and you'll be "about" right. But the correct formula does call for TAS, not GS.

A proper formula would be GS/100 (not 200).

I disagree. The formula I've quoted works in all aircraft I've ever flown, and I've seen it in print (Mental Math for Pilots, IFR Magazine) more than once. Additionally, the concept appears in other forms. Transport Canada's Flight Training Manual advocates a DME arc intercept technique using 0.5% of GS (or TAS) for a standard rate (they call it 'Rate One') turn and 1.0% for a 1/2 standard rate turn. Since the Chickenhawk isn't likely to be going fast enough to ever require a half-standard rate turn for anything, 0.5% of 120 knots = 0.6nm (exactly the same result as the formula I published above.) An airplane with a groundspeed of 200 knots would require 1nm (0.5% of 200 = 1nm) for the lead-in, etc.

Another common rule of thumb is to use a 0.5nm lead-in for groundspeeds below 150 knots. It's not quite as precise - you'll overshoot a bit at the higher speeds - but it essentially works, and is pretty much in line with the formula I provided. As indicated above, add .1 or .2 to account for the roll-in. Add more if you're above 200 knots, but most of us aren't intercepting DME arcs much above that speed, whether we're flying jets or pistons.

In summary, your suggestion of TAS (or GS) / 100 would be accurate for half-rate turns, not standard rate. The piston driver would grossly undershoot, every time.

Don't believe me? Do the math: How long does it take to do a standard rate, 90 degree turn (including lag time to initiate the turn)? How far does an aircraft with a GS of 120kts travel in that time?

The math isn't as straightforward as you seem to think, and doesn't support your inferred conclusion. Take two airplanes flying 120 knots straight ahead; one begins a standard rate, 90 degree turn while the other continues. Start a stopwatch. The speed of the turning airplane relative to the level-wing airplane drops from identical (120 knots) at the beginning of the turn to zero at the thirty second mark. Had he continued straight ahead for one minute, he'd travel about a mile. But since he made the standard rate turn, he only covers about half that relative distance - 0.5nm. I.e., median average of 120 knots at the 0 second mark and 0 knots at the 30 second mark = 60 knots, which would cover 0.5nm.

FWIW- A 90 degree intercept seems to be the most common entry for DME arcs.

Right. But I'd suggest 80 degrees when intercepting from the outside, which is how most of us use DME arcs, most of the time.

Best regards,

-Ryan

 

[fokkers&beer]

It aint rocket science. I typically shoot 2 arcs a day in SE Alaska. In the Fokker I lead it by about 2 miles. How did I come up with this?, elementary my friend, I shot a butload of arcs. In the ATR however because of the slow reaction and Autopilot, I lead three miles. It just takes a few arcs to figure out how fast the aircraft will roll 90 degrees. There is probably a formula, and I will let you UND guys explain how it works.

I myself use 2 or 3 miles depending on the AC.

 

[prpjt]

No, it's TAS.

Since the Chickenhawk isn't likely to be going fast enough to ever require a half-standard rate turn for anything, 0.5% of 120 knots = 0.6nm (exactly the same result as the formula I published above.) An airplane with a groundspeed of 200 knots would require 1nm (0.5% of 200 = 1nm) for the lead-in, etc.

-Ryan

Using the Chicken hawk example, lets approach this a different way. Let's say you have it pulled back to 80 knots TAS. Here we go.

Plane #1 approaches the arc with a 70 knot tailwind. Your method has you starting the turn 0.4 nm out. Your aircraft is travelling 150 knots toward the intercept point. I contend that you'll blow through it.

Plane #2 approaches the arc with a 70 knot headwind. Your method has you starting the turn 0.4 nm out. Your aircraft is approaching the interrcept point at 10 knots. Forget starting a turn at .4 nm, You would have to fly for almost 2.5 minutes straight ahead just to get to the arc.

It's GS.

 

[A Squared]

It's GS.

Yeah, it's groundspeed, obviously, as your example shows pretty conclusively. I guess Pilot_ryan has never really though it through all the way.

 

[HMR]

Yeah, it's groundspeed, obviously, as your example shows pretty conclusively. I guess Pilot_ryan has never really though it through all the way.

Oh, C'mon guys... when do we ever see 70kt winds?

Best Regards,

-HMR

 

[bubbers]

The only way to fly any approach is to keep it as simple as you can. DME arcs and NDB approaches don't require a lot of formulas or rocket science thinking. The more complicated you make it the better your chances of missing the forest for the trees. The pilots I have seen screw things up most can not prioritize what is important right now and what is not. No DME arc will kill you if you get a half mile off. Starting a descent too early will. Pay attention to that. Flying airplanes safely is designed for average pilots with descent judgement.


Some of the most intelligent pilots I have flown with can figure out the exact FAA perfect way to do something when problems happen but don't have the common sense to just fly the plane and not dig yourself into a hole so when you crash you were 100% legal.

 

[Vandal]

Holy crap way to confuse the guy...To intercept turn to put the RMI needle pointing at you left or right wing. Then watch dme, if it is going up, more left turn, if it goes down more right turn. (For a counter clockwise arc) As simple as that...just watch the DME.

 

[NYCPilot]

From "The Instrument Flying Handbook"

DME Arcs


A .5 NM lead is satisfactory for groundspeeds of 150
knots or less. At higher groundspeeds, use a proportionately greater lead.

Continue the turn for approximately 90 degrees.

During the last part of the intercepting turn, monitor the
DME closely. If the arc is being overshot (more than 1.0
NM), continue through the originally-planned roll-out
heading. If the arc is being undershot, roll out of the turn
early.

When flying a DME arc with wind, it is important that you
keep a continuous mental picture of your position relative to
the facility. Since the wind-drift correction angle is constantly changing throughout the arc, wind orientation is important. In some cases, wind can be used in returning to the desired track. High airspeeds require more pilot attention because of the higher rate of deviation and correction.

Maintaining the arc is simplified by keeping slightly inside
the curve; thus, the arc is turning toward the aircraft and interception may be accomplished by holding a straight course. If you are outside the curve, the arc is “turning away” and a greater correction is required.

To fly the arc using the VOR CDI, center the CDI needle
upon completion of the 90 degree turn to intercept the arc. The aircraft’s heading will be found very near the left or right
side (270 degree or 90 degree reference points) of the instrument. The readings at that side location on the instrument will give primary heading information while on the arc. Adjust the aircraft heading to compensate for wind and to correct for distance to maintain the correct arc distance. Re-center the CDI and note the new primary heading indicated whenever the CDI gets 2 – 4 degrees from center.

With an RMI, in a no-wind condition, you should theoretically
be able to fly an exact circle around the facility by
maintaining an RB of 90 or 270 degrees. In actual practice, a series of short legs are flown.

1. With the RMI bearing pointer on the wingtip reference
(90 or 270 degree position) and the aircraft at the desired DME range, maintain a constant heading and allow the bearing pointer to move 5 to 10 degrees behind the wingtip. This will cause the range to increase slightly.

2. Turn toward the facility to place the bearing pointer 5 –
10 degrees ahead of the wingtip reference, then maintain heading until the bearing pointer is again behind the wingtip. Continue this procedure to maintain the approximate arc.

3. If a crosswind is drifting you away from the facility, turn
the aircraft until the bearing pointer is ahead of the wingtip
reference. If a crosswind is drifting you toward the facility,
turn until the bearing is behind the wingtip.

4. As a guide in making range corrections, change the RB
10–20 degrees for each half-mile deviation from the desired arc. For example, in no-wind conditions, if you are 1/2 to 1
mile outside the arc and the bearing pointer is on the
wingtip reference, turn the aircraft 20 degrees toward the facility to return to the arc.

 

[Singlecoil]

.5% of your ground speed works for a Cessna, but in higher speed aircraft you need a different approach. I got tired of hearing these different rules of thumb that didn't work in different scenarios, so I fired up and Excel spreadsheet so I could get a grip on different leads for different speeds, bank angles in the turn, and also rate of roll into the turn, which is an important factor.

Radius of turn is GS squared divided by 11.26 times the tangent of the angle of bank. Smaller aircraft use standard rate, while the bigger stuff uses 25 or 30 degrees of bank. That will give you the radius of the turn, but then you have to figure in how long it took you to establish the bank, what distance you travelled in that interval, and add that in to your lead.

Basically, for a 90 knot groundspeed, you would need .6 for a 5 second roll in to a 15 degree bank, or .7 for a 10 second roll in.

At 200 knots and 25 degrees of bank, you would need 1.5 for a 5 second roll in and 1.8 for a 10 second one. 30 degrees of bank yields 1.3 and 1.6, respectively.

At 250 knots and 25 degrees of bank, you would need 2.3 for a 5 second roll in and 2.7 for a 10 second one. 30 degrees of bank yields 1.9 and 2.3, respectively.

The .5% rule would be way off, yielding 1.25 miles in the 250 knot scenario.

Fire up Flight sim and test those, they will be right on. If anyone wants a copy of the spreadsheet, I'll email it to you, just pm me.

 

[atldc9]

1. Direct to the fix.
2. Select the arc transition.
3. Select 1R on the FMS, Confirm, enter, AP1.
4. Watch the white airplane eat the magenta line. Even figures out its own wind correction, and tells you if it is off by more than 0.06 miles.
5. Call for coffee.

Now that is the proper procedure.

 

[five-alive]

1. Direct to the fix.
2. Select the arc transition.
3. Select 1R on the FMS, Confirm, enter, AP1.
4. Watch the white airplane eat the magenta line. Even figures out its own wind correction, and tells you if it is off by more than 0.06 miles.
5. Call for coffee.

Now that is the proper procedure.

touche