Hold DME figures

[devilsmartinirocks]

Hi chaps/chapesses,

Can anyone be kind enough to inform me where I can find out what the distance in nm should be from an NDB at 2000 feet, 3000 feet, etc, at the thirty degree 'Gate' position (no wind), when utilising NDBs for holding? Also, when applying two or three times the single drift correction (for wind), on the outbound leg, is there any way of calculating how this affects the distance at the thirty degree 'Gate'? I believe, by applying these wind calculations, the 'Gate' can move, but only by as much as the value of single drift. Correct? Lastly, when directly overhead an NDB, is the nm displayed on the DME equal to a proportion of the actual height, i.e. at 6000' the display shows 1.1nm, at 3000' the display shows 0.5nm?

This information, together with the 'Gate' and the mighty stopwatch, will, hopefully, help me realise just exactly where I am on the outbound leg of the hold, enabling me to work out just what the hell I need to do to allow me to get back onto the inbound track for a "reasonable amount of time" before reaching the beacon.

Many thanks

DMR

[Dr Eckener]

You cannot use DME to judge a hold. There are too many variables. I am sure you could work out for yourself the distances in still wind at a paricular speed and altitude, but it would be academic and of no value in flying a real hold.

[FlyingForFun]

Dr Eckener is correct.

Lots of people get very stressed about the gate, and forget what they are actually trying to achieve. The aim of the gate, as well as just about everything else you do in the hold, is to get established on the inbound track. The gate is just a starting point to help you achieve this. Techniques like varying the gate (by up to single drift) are used by many people. My opinion, though, is that they just make things over-complicated.

The first time around the hold, if you were to completely ignore the gate and just see what happens on the inbound turn, you would soon find out if your outbound heading (triple-drift or double-drift or whatever) was working. If it is, then leave everything as it is. If it's not, then adjust the outbound heading next time around the hold. That alone, in the real world, would be enough to be able to fly holds which are perfectly good enough.

There is a lot of extra information which can help you with situational awareness throughout the hold, and the gate is the most important of these. But, as you say, it can vary depending on the wind, and all we know about the wind before we've flown the hold a few times is what the forecasters have told us, which quite possibly isn't all that accurate. So, whilst it is important (to IR examiners, at least) to monitor the gate, and turn in or out if you are clearly not achieving the gate, bear in mind that if it is one or two degrees out, it really doesn't matter - so long as the inbound turn is working.

What I see a lot of students doing is working very hard to achieve the gate, then turning inbound and finding that they are too close or too far out. Next time around the hold, they work very hard again the achieve exactly the same gate, and, surprise surprise, are too close or too far out. This is what I mean about "getting stressed about the gate" - it is only a tool to help you achieve the inbound track when you haven't quite figured out the exact outbound heading. If you know that the heading you used last time left you too far out, then turn in next time, and if you've got the capacity then look for a slightly different gate too.

As for DME, that depends on so many different factors that it's rarely very useful. The only thing it might be useful for is a gross-error check. Know roughly what the DME should say as you turn inbound (this will depend on the speed of your aircraft). Then, if you get your headwind/tailwind calculations the wrong way round, for example, you'll soon realise that the DME looks very wrong. But if it's 0.2d out from what you'd expect don't worry about it, because that kind of variation is quite normal.

Hope that helps,

FFF
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[bookworm]

Can anyone be kind enough to inform me where I can find out what the distance in nm should be from an NDB at 2000 feet, 3000 feet, etc, at the thirty degree 'Gate' position (no wind), when utilising NDBs for holding? Also, when applying two or three times the single drift correction (for wind), on the outbound leg, is there any way of calculating how this affects the distance at the thirty degree 'Gate'? I believe, by applying these wind calculations, the 'Gate' can move, but only by as much as the value of single drift.


When not using a perfectly co-located DME, and many of them are separated by a substantial distance on the airfield, it's impossible to generalise. When using a perfectly co-located DME, the slant range issues are not as serious at low level as one might expect.

Say at 120 KTAS, one minute still air is 2 miles, and a rate one turn diameter is 4/pi = 1.27 miles. The arctan of the ratio of the two is 32 degrees, which is where that gate comes from. The 2D-distance from the beacon is 2.36 miles. That's a slant range of 2.41 miles at 3000 ft and 2.56 miles at 6000 ft.

In a crosswind situation, the wind will blow you for a minute in the turn.
Thus for a 20 knot xwind, the turn diameter changes by 1/3 mile, and the gate bearing becomes 25 degrees (2D-dist 2.21 miles) or 37 degrees (dist 2.56 miles). So that gives you an idea of how the gate bearing changes with xwind -- about 6 degrees for 20 knots equivalent to 10 degrees single drift.

That should give you an idea of the variation you might expect, so that you can be aware of gross errors.

[tbc]

FFF, all noted and generally agreed however it makes a bit of a difference if you only get one hold on the test.

[Dr Eckener]

tbc,

You can only use the forecast wind until you get into the air to assess it. You should however be able to see if the wind is roughly as forecast enroute. How does your drift and grounspeed compare to your plan? This will give you greater confidence in doing the hold.

As for the hold itsself, you can only change 3 things if they don't work out - heading outbound, the gate and time outbound. Just make sure you learn from the previous hold and make the correct assessment.

[tbc]

Dr E,

I have no argument with what any of this, but most are suggesting you get more than one hold - you may and I would suggest a fair and decent examiner would want to see each hold get better, but I have had IR candidates who are given only one hold and not surprisingly they failed that part of the test.

When I spoke with the IRE afterwards he advised me that one hold is all they have to see - so get it right first time is what he was suggesting.

[the dean]

it has become less important...and in JAA land...it used ti be back over the hold +/- 20 seconds ( whether you use a 4 minute hold from overhead or 3 minute from 90 degrees opposite the beacon on the outbound leg ( +/- ) the correction...

if it stays largely within the printed hold ( it would be very nice to be exactly on the correct QDM..!!..that is acceptable to me..:ok:

and remember...the entry and once round is not 'IN' the hold as such...it is the entry ...( even if the entry has been direct...most examiners would agree ) so a lot can be learned from the entry and the sharp guy/girl can actually seek to establish on the QDM in that first round rather than 45 seconds out and turn inbound for the beacon...but that depends on whether the entry has been direct or not...

the dean.

and one hold may be all he needs to see but that does'nt mean its all he can give...thats at his discretion...