"Rule of thumb" part of the LO for radio nav?
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"Rule of thumb" part of the LO for radio nav?
This JAA sample question illustrates the issue;
The rate of descent required to mainain a 3.25 degree glide slope at a groundspeed of 140 kt is approximately:
a) 760 ft/min
b) 850 ft/min
c) 670 ft/min
d) 700 ft/min
The mathematically correct answer is 805 ft/min which makes the question unanswerable using the most accurate means of calculation. I have been warned that my NAA will ask questions like this on radio nav where you have to use the "rule of thumb", "1:60 rule" or similar.
I had a quick look at the learning objectives and I couldn't find anything on alternative calculation methods. Have I missed something here or is a question like the above-mentioned considered to be outside of the LO?
The rate of descent required to mainain a 3.25 degree glide slope at a groundspeed of 140 kt is approximately:
a) 760 ft/min
b) 850 ft/min
c) 670 ft/min
d) 700 ft/min
The mathematically correct answer is 805 ft/min which makes the question unanswerable using the most accurate means of calculation. I have been warned that my NAA will ask questions like this on radio nav where you have to use the "rule of thumb", "1:60 rule" or similar.
I had a quick look at the learning objectives and I couldn't find anything on alternative calculation methods. Have I missed something here or is a question like the above-mentioned considered to be outside of the LO?
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Well, I'm sure there's a nice simplification for each question that might come up. I'm confident using the calculator and want to know if the learning objectives stipulates any other method of calculation. At the end of the day I want to be sure that I pass the test (or have a strong case if I need to appeal).
For 3 deg, it's half the GS plus a zero. So for 140 knots G/S it's 700 ft/min. 3.25 is just under 10% steeper, so descent rate at 140 knots G/S should be just under 10% greater, i.e. a tad under 770 ft/min. Nearest answer is 760.....
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You have to learn, good and early, that the JAA just LOVE the 1 in 60 rule.
So, if there's a possibility that the 'ball park' answer using the 1 in 60 rule' gives a less accurate (but neater) answer than classical trigonometry, you can bet your boots that the JAA will go for it as the preferred answer in the JAA question bank (except on the rare occasions when they don't).
Don't ask me to justify this stuff.
It;s just the way it is is if you're a wannabe and you want to become a pilot.
So, if there's a possibility that the 'ball park' answer using the 1 in 60 rule' gives a less accurate (but neater) answer than classical trigonometry, you can bet your boots that the JAA will go for it as the preferred answer in the JAA question bank (except on the rare occasions when they don't).
Don't ask me to justify this stuff.
It;s just the way it is is if you're a wannabe and you want to become a pilot.
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The clue for me is the word approximately. These are the sort of MDR calculations that will give you an approx answer. If you get yourself on a 3.25 degree glidepath, and start going low with your "accurate" RoD, you telling me you aint going to adjust it because your calculator said 805 !!!!!
It's not going to be 700, as that is the answer for a 3 degree glidepath at 140 kts. So work around it and come out with the approx answer, as asked for.
I realise it's an exam question and you want to have a black/white answer but try and think of what the question is actually getting at.
It's not going to be 700, as that is the answer for a 3 degree glidepath at 140 kts. So work around it and come out with the approx answer, as asked for.
I realise it's an exam question and you want to have a black/white answer but try and think of what the question is actually getting at.
Last edited by FFP; 23rd Oct 2004 at 12:24.
It's still a poorly worded question. Good question construction requires the method(s) to be used (or not used) be stated where an otherwise correct/accurate method would not give the desired response.
Alternatively the option differentiation must be large enough that all correct methods resolve to the correct response.
Alternatively the option differentiation must be large enough that all correct methods resolve to the correct response.
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Hy,
the real 1:60 rule:
Height: (glidepath angle*range)/60*6076
This can be simplified to:
Height=glidepath angle*range*100
Rate of descent:
ROD: (glidepathangle*1)/60*6076* groundspeed/60
the real 1:60 rule:
Height: (glidepath angle*range)/60*6076
This can be simplified to:
Height=glidepath angle*range*100
Rate of descent:
ROD: (glidepathangle*1)/60*6076* groundspeed/60
Subsidence
,
Wondering why you bother with the 6076? There are enough other approximations behind the 1:60 method that the 76' is irrelevant. I'd be inclined to call it 6000' & be done with it. Just curious, that's all.
,
Wondering why you bother with the 6076? There are enough other approximations behind the 1:60 method that the 76' is irrelevant. I'd be inclined to call it 6000' & be done with it. Just curious, that's all.
Last edited by Tinstaafl; 24th Oct 2004 at 22:49.
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subsidence - if you're using the conversion of nm to feet, use the UK definition which is 6080. However, as Tinny says, don't bother.
For kts to ft/min conversion just multiply by 100 - you are then using 6000 feet per nm and 60 minutes per hour. The error is about 1.3%, and considering the error of teh 1:60 rule is between 4 and 5% that is adequate!
Notice this is the source of the "rule of thumb" of 5 times groundspeed, which in various forms most people are recommending.
RoD = [Groundspeed (kts) x 100 (kts to ft/min) x 3 (typical glideslope angle)] / 60
Since 300/60 is 5, you come up with the result G/S multiplied by 5.
For this question just go back to the original equation and substitute 3.25 for the 3. The answer comes out as 758.3, close enough for government work!
For kts to ft/min conversion just multiply by 100 - you are then using 6000 feet per nm and 60 minutes per hour. The error is about 1.3%, and considering the error of teh 1:60 rule is between 4 and 5% that is adequate!
Notice this is the source of the "rule of thumb" of 5 times groundspeed, which in various forms most people are recommending.
RoD = [Groundspeed (kts) x 100 (kts to ft/min) x 3 (typical glideslope angle)] / 60
Since 300/60 is 5, you come up with the result G/S multiplied by 5.
For this question just go back to the original equation and substitute 3.25 for the 3. The answer comes out as 758.3, close enough for government work!
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A question similar to the above-mentioned appeared on the exam. The question could be solved both using the 1 in 60 rule and using trigonometry.
However, can anyone enlighten me what they mean by;
Flying on a 120 radial on a VOR. Course is 340 and magnetic heading 070. What is the dev. bar in relation to the aeroplane?
Correct answer: behind
Anyone explain, please?
However, can anyone enlighten me what they mean by;
Flying on a 120 radial on a VOR. Course is 340 and magnetic heading 070. What is the dev. bar in relation to the aeroplane?
Correct answer: behind
Anyone explain, please?
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This doesn't have anything to do with the 1 in 60 rule but requires an understanding of VOR's.
It's not easy to explain in words but I will do my best.
If you are on the 120 radial, then to track to the VOR the track would be the reciprocal of that, 300. You need a track of 340 to the VOR so you need to be on the reciprocal of that, the 160 radial. As you are heading 070 you are tracking away from the radial you require so the display will show it as behind you.
Try to draw it out and it becomes much clearer.
NH
It's not easy to explain in words but I will do my best.
If you are on the 120 radial, then to track to the VOR the track would be the reciprocal of that, 300. You need a track of 340 to the VOR so you need to be on the reciprocal of that, the 160 radial. As you are heading 070 you are tracking away from the radial you require so the display will show it as behind you.
Try to draw it out and it becomes much clearer.
NH
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You are:
On the 120 radial
Heading 070
You have:
Course 340 selected on the VOR
The instrument will show a TO flag and show fly left deviation of >= 10 degrees.
The heading is irrelevant as the VOR shows the same indication regardless of the direction you are pointing in. If the answer is "behind" then they must somehow be referring to the orientation of the devation bar on an HSI not a VOR.
On the 120 radial
Heading 070
You have:
Course 340 selected on the VOR
The instrument will show a TO flag and show fly left deviation of >= 10 degrees.
The heading is irrelevant as the VOR shows the same indication regardless of the direction you are pointing in. If the answer is "behind" then they must somehow be referring to the orientation of the devation bar on an HSI not a VOR.
