intercepting radials and flying DME arcs with a jet

[N2334M]

Hello.

I am currently practising IFR flying with MS Flight Simulator 2004 and the Boeing PMDG 737-800, with 60T TOW. I normally maneuver at 250KIAS using the bank limiter at 30°AOB.
I cannot afford to go in a real sim, so that's all I have. I need some advise to know if what I am doing is correct or not...

I have some questions:

1. when starting a DME arc with a jet, how earlier do you start your turn ?
Ex. if I am at 250KIAS @FL200 with 0 wind, and my GS readout is 336KT, I noted that I have to start my turn at 4NM before the required DME arc. Could it be possible or is it a non precision of the Flight Simulator ?
How would you calculate your anticipation to start the turn given a groundspeed ? (I was given a formula for prop aircraft, but apparently it does not work for jet I think due to the radius of turn).

2. how much bank angle would you use for maneuvering during DME arcs, or intercepting radials ? How fast ? is 250KIAS too much ?

3. how much anticipation would you use when leaving the arc and intercepting the radial inbound or outbound ? I have seen that I have to start my turn at least 20° earlier if not I overshoot very badly !

4. Do you know where can I find some rules on how to perform interceptions ?

Thanks a lot !

Best regards,
N2334M.

[Astra driver]

It sounds like you are on the right track.
As a rule of thumb I use 10% of my ground speed for my DME intercepts, ie 200 knots GS, turn to intercept 2 DME prior.

The only thing you should keep in mind is that if a DME arc is flown as part of an approach procedure you should fly it at approach speeds, say between 200 and 180 KIAS.

If you are flying the arc as part of a departure procedure then the 250 knot speed would be more realistic unless the procedure has a speed limit posted.

As far as leading the arc inbound, most approach plates have a "Lead in" Radial marked on the arc in order to give sufficient time to turn in on the inbound course. If you are following the procedure and are still overshooting it is probably because you are going too fast. In most cases your speed should be down to between 170 and 210 knots with flaps at the first approach setting at this point.

Under no circumstances should your bank angle exceed 30, and if your speed is getting down to 180 knots or less without full flaps I would further limit bank angle to less than 20.

Just a personal technique, I am sure others would work equally well.

AD

[PropstoMAX]

Astra Driver's method works very well, although using 1% of GS as an anticipation and not 10%.. 20dme would be abit on the early side for a GS of 200kts!!:confused:

[Astra driver]

Brain fart!

yep, I meant 1% not 10%, thanks proptomax !

[skycoolie]

Just as a matter of interest, I used to try and do some IRR practise on MS Flight Sim (Professional), just procedural stuff, holds, letdowns, intercepts and the like.

What intrigued me was that the sim model MS Flight Sim uses cannot duplicate a Rate 1 turn. Try it some time. Set up 180 kt, 25 degrees of bank and time the turn. Instead of achieving 3 degrees per second, it will be less than 2! Pretty useless for practising holding and intercept procedures. Your turns are going to be much wider than they would be in the real universe!

I used the rule of thumb for a rate 1 of TAS divided by 10, plus 7 for the angle of dangle, and even selected the MS display to TAS to make it as accurate as I could.

At first I thought it was just the speed of my computer, but I have one of the fastest and it doesn't seem to matter which other computer I use. Whoever modelled this particular function in the game failed to get this bit of physics right.

I even tried to talk to someone from MS but when they phoned back, the guy they had answering my question had no idea what I was talking about.

So there, you go. I wouldn't put too much faith in it.

[OzExpat]

(coincidentally you'll find most Apch plates annotate "lead radials" which are 7 before!)
Pans Ops recommends 2NM of lead for a turn off an arc, regardless of the arc radius, and I'd be most surprised if TERPs is any different to that. Thus, the lead radial will differ according to the different arc radii that you might encounter.

If you only ever encounter a 15 DME arc, the 7 radials will be about right, but bear in mind that Pans Ops sets a minimum arc radius of 7 NM. I've seen many approach charts that use a 10 DME arc, so the 7 radials of lead will be too much.

[ifleeplanes]

On the lead radial theory the 10D arc would give 6 radials per mile and thus at 180kts GS the 1.8 miles would give 11 radial lead for the turn. Not the seven, the seven is just the 15D intercept.

Its a good theory and a useful tool. By this stage its all a guesstimate anyway.

[OzExpat]

Yes ifleeplanes, in my previous post I was concentrating on the standard 2NM of lead, rather than the actual radius of turn of an aircraft. I know the radius of turn of the aircraft I fly, in various speed configurations, and can therefore pro-rate the degrees of difference between the lead radial and the required track, so that I turn at the radial that equates to my radius of turn, whether that's before or after the lead radial shown on the chart.

It has to be said though that if we assume that TAS = GS for a given set of conditions that equates to a GS of 180 knots, the radius of turn for 15 degrees of bank is 1.8 NM. At 20 degrees of bank, it's 1.3 NM and at 25 degrees, which I mostly use, it's 1.01 NM. These differences will certainly make a difference to the lead radial that would actually be used. Note that they are all less than the 2 NM lead provided on the chart.

[keithl]

OzEx - Can you give me a reference for that minimum 7nm, please - I'm currently arguing against a design which proposes a 4 dme arc but I couldn't find a minimum figure.

[Rainboe]

skycoolie- your comment about Rate 1 turns needs answering. You can do great Rate 1 turns in a Cherokee, but at somewhere roundabout the 150 kt (or less) mark, it is no longer possible to do a Rate 1, 3 deg/sec turn anymore and still restrict your bank to 25 degrees. Concorde in supersonic cruise could probably still not do a Rate 1 turn even at 90 degrees bank. It is a function of speed. Big jets just use 25-30 degrees and leave it at that- it gives far less than Rate 1, so the MS sim is quite correct.

[Soulman]

You may also like to enter the navaid in the FIX page of the FMC and create a DME circle by entering /XX in the scratchpad @ LSK3L, where XX is the number of nm from the station - this way you also get a visual of the DME arc.

You may have already done this - but I thought it might help.

[normal_nigel]

Do a map check.

Select DME ARC approach in the FMC and press LNAV.

Once on the arc use heading.

Works every time

[skycoolie]

I appreciate that Rainboe. That's why I used 180kt. A rate 1 turn at a TAS of 180 kt means turning at 25 degrees of bank. A jet is limited to 25 degrees of bank maximum for IR procedures. A rate 1 turn (3 degrees per second) cannot be achieved in MS Flight sim at the same speed of 180 kt when it should be able to.

At 120 kts, a rate 1 turn requires 19 degrees bank

At 250 kts you would need 32 degrees of bank

At 450 kts, you wouldn't need 90 degrees, but rather 52 degrees of bank

And all of them are supposed to traverse 180 degrees of a circle in 1 minute. MS Flight Sim is unable to duplicate this. It's radius of turn is way to wide to be realistic, at any speed!

[OzExpat]

keithl... I'm not online at work and don't keep my Pans Ops book at home. Will have to check first thing Monday morning and let you know on Monday afternoon. Sorry about that.

Confess that I haven't needed to look at that aspect for a really long time. Last time I had need of it was when I designed an 8 DME arc for the NDB/DME approach at Mount Hagen (AYMH). That would've been about 1988 or so! :eek: God I'm old!!!

I just hope that it's still in there somewhere. With all the latest changes and reorganisations of the book, it might've been omitted. :uhoh: I probably should've checked before talking about the 7 NM limit but, like I say, I don't keep Pans Ops at home. :ugh:

In any event, I think that you're better off basing your argument on the "flyability" of a procedure that incorporates a 4 DME arc. Don't think I'd fancy my chances with it! :ooh:

I'm guessing that your problem relates to some sort of obstacle-limited departure procedure? If so, you've got the additional argument of a 15 degree bank angle limit, which might make it real difficult to turn onto the arc and then maintain it without a massive speed restriction.

[Rainboe]

Coolie- I seem to recall from my training days that even 180kts was too fast to achieve a Rate 1 turn at 25 degree bank. I may well be wrong, but I thought I'd left Rate 1s behind with the Cherokee. It was rather a long time ago!

Do you know where can I find some rules on how to perform interceptions ?

You're going to get as many different opinions as people answer! I would suggest this- flying procedures should be at minimum flaps up speed until you start thinking about approach. This means something like 230 kts on a 747 and 210 kts 737. 250 is too fast- turns far too big. The small increase makes a big difference. In holds in a light aeroplane, I remember as you were coming around to the inbound course, at 90,60,30 degrees to go, we used to look for lead to inbound course of something like 25,15,5 degrees to go, allowing for wind- I can't remember the exact numbers. That was under very precise training. Now I feel KISS (keep it simple, stupid). I find trying to do fancy mental calculations in your head whilst trying to align onto a course too complicated- you can easily turn the wrong way or mistake the right track. Just make an estimation of turn radius/wind effects, see how it works out and use that to alter the next time around. Turn radius doesn't really help you when you are looking at an ADF needle.

[keithl]

Thanks, OzEx, pure laziness on my part. Because I've now found it in Vol II - I was just looking in Vol I when I posted my query.
You are quite right (so have a clear conscience for the w/e!). 7nm for Cat A, 5nm for Cat H.

Yes, the proposal is unflyable, but I like to have Chapter and verse ready as well.