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NDB Holds
Hi,
I'm studying for my IR and having some trouble in calculating the outbound times. Has anyone got any easy way of calculating the times. |
#2
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NDB Holds
I'm studying for my IR and having some trouble in calculating the outbound
times. Has anyone got any easy way of calculating the times. What I do in real life is to time the inbound, and then whatever the error is on the inbound, I apply it to the outbound the opposite way. It may take a couple of turns to get it right since the relationship is not linear, but for small wind speeds compared to aircraft speeds, it seems to work well enough. Time outbound one minute. Time inbound. Say it's forty seconds. That's twenty seconds too little, so next time go outbound a minute and twenty seconds. Time the inbound. Say it's fifty-five seconds. Next time go outbound another five seconds - that is, go out a minute and twenty-five seconds. Iterate until happy. If interested, the actual formula is: (where v is velocity, t is time, and d is distance) v = d / t or d = v t The two distances (along the inbound course) will be the same, to first order, so set them equal. d = (v1)(t1) = (v2)(t2) Rewrite v1 as v+w and v2 as v-w (where w is wind speed) Rewrite t1 as t and t2 as t+d (where d is the difference between one minute (t1) and the inbound.) We end up with (2w)/(v-w) = d If I didn't mess anything up. (this doesn't take the crosswind's effect on the half circle sizes into effect). Jose -- "Never trust anything that can think for itself, if you can't see where it keeps its brain." (chapter 10 of book 3 - Harry Potter). for Email, make the obvious change in the address. |
#3
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NDB Holds
On 10/27/06 14:35, Alan Burke wrote:
Hi, I'm studying for my IR and having some trouble in calculating the outbound times. Has anyone got any easy way of calculating the times. Alan, See Jose's answer for the timing. All I wanted to add was that you should guestimate the crosswind correction before entering the hold, and apply this on the first leg. Otherwise, the crosswind may really mess with the timings. Remember to adjust for the crosswind on the inbound leg as well ;-) I would have guessed your question would have been more like "how do you tell when to *begin* timing on the outbound leg". .... perhaps that's still coming ;-) I hope that you'll post more about your IR adventures. I sure would like to read about them and participate in answering questions, etc. Best Regards, -- Mark Hansen, PP-ASEL, Instrument Airplane Cal Aggie Flying Farmers Sacramento, CA |
#4
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NDB Holds
Mark Hansen wrote:
On 10/27/06 14:35, Alan Burke wrote: Hi, I'm studying for my IR and having some trouble in calculating the outbound times. Has anyone got any easy way of calculating the times. Keep in mind, as you progress with this, that the "rule" is to not exceed one minute inbound to the NDB. |
#5
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NDB Holds
Thanks guys for the response. What I suppose I am asking if how do you work
out the times (pre-planning) of the hold. I.e if you know the wind direction and speed for the hold, I've worked out the single drift and in my notes it says to work out the timings use 75% of the head / tail wind factor. This is where I get stuck. How do you work that out?? "Sam Spade" wrote in message ... Mark Hansen wrote: On 10/27/06 14:35, Alan Burke wrote: Hi, I'm studying for my IR and having some trouble in calculating the outbound times. Has anyone got any easy way of calculating the times. Keep in mind, as you progress with this, that the "rule" is to not exceed one minute inbound to the NDB. |
#6
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NDB Holds
In article ,
"Alan Burke" wrote: Thanks guys for the response. What I suppose I am asking if how do you work out the times (pre-planning) of the hold. I.e if you know the wind direction and speed for the hold, I've worked out the single drift and in my notes it says to work out the timings use 75% of the head / tail wind factor. This is where I get stuck. How do you work that out?? Don't try to "work it out". Your primary job is to fly the airplane, not work out complex geometry problems. As you're approaching the hold, take a WAG (Wild Assed Guess) at wind strength. In most common trainers, you'll be holding at about 90 kts. A really big headwind component on your outbound leg would be 30 kts, making your outbound groundspeed 60, and your inbound groundspeed 120 (double your outbound). So, that's pretty much an upper limit, and gives you your first WAG: with a really honking headwind outbound, fly the outbound leg for 2 minutes. Estimate down from the with a moderate headwind outbound, try 1.5 minutes. With little or no wind, try 1 minute. With a moderate tailwind outbound, try 50 seconds, and with a really honking tailwind outbound, try 40 seconds. If you're trying to get things any more accurate than that, you're doing way too much work. It's better to fly your first outbound leg a little bit long rather than a little bit short. If you under-estimate how long to fly outbound, by the time you complete your inbound turn, you can be almost on top of the NDB and not have enough time to re-intercept the inbound bearing before you reach station passage. When that happens, you tend to just keep getting more and more confused and lost. But, the real way to do NBD holds is to tear that piece of crap ADF out of the panel and train in a GPS-equipped airplane. If your school doesn't have GPS in their trainers, find another school. |
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NDB Holds
Alan,
my notes it says to work out the timings use 75% of the head / tail wind factor. Hmm. Why not 100 percent? The way I would do it is to figure out the percentage the head/tail wind is of my IAS (TAS, if you want to get precise). Then, I would correct 1 minute by that percentage. From your notes, your supposed to take 75 percent of that percentage. In reality, this takes way too much brain power away from controlling the aircraft. Make intelligent guesses. E.g. your holding speed is 90 knots, your wind component is an estimated 10 knots, so you try a 10 seconds correction. -- Thomas Borchert (EDDH) |
#8
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NDB Holds
Thomas Borchert wrote:
Alan, my notes it says to work out the timings use 75% of the head / tail wind factor. Hmm. Why not 100 percent? The way I would do it is to figure out the percentage the head/tail wind is of my IAS (TAS, if you want to get precise). Then, I would correct 1 minute by that percentage. From your notes, your supposed to take 75 percent of that percentage. In reality, this takes way too much brain power away from controlling the aircraft. Make intelligent guesses. E.g. your holding speed is 90 knots, your wind component is an estimated 10 knots, so you try a 10 seconds correction. At 90 knots it all is just b.s. to try to placate some anal instructor or check airperson. The holding pattern protected airspace is designed for a jet aircraft. |
#9
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NDB Holds
Alan Burke wrote:
Hi, I'm studying for my IR and having some trouble in calculating the outbound times. Has anyone got any easy way of calculating the times. Just finished mine over here in Germany. We were supposed to learn it, however our flying clubs' Pipers have 2 GNS 430, with the excellent "pictures"... Anyway: Here is an easy method, if you have lets say 3+ miles to go in a 100kts airplane: Time: Subtract/Add the Wind-speed in seconds from/to your 1 min as follows: Wind angle 30° full speed. Wind angle 30..60° half speed. Wind angle 60° no correction. Example: Wind 300/20, Course 290°. Wind angle=10°, that is "full speed" so ADD 20seconds (20kts) to your 1 min. By the way, that works with VORs as well. Of course, in the real world, in a small plane...who gives a f* ? You'll stay almost always in the protected area, no matter what heading you fly.... ;-) Cheers, Kusi |
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