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ATPL theory questions

Old 20th Jan 2013, 18:49
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Hi

Q.1. When switching on the weather radar, after start-up a single very bright line appears on the screen. This means that the:

scanner is not rotating
scanning of the cathode ray tube is faulty

Q.2. On switching on the AWR a single line appears on the display. This means that:

the CRT is not scanning
the antenna is not scanning

Whats the main difference between the two?
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Old 21st Jan 2013, 09:47
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Long time since I did this but I THINK its 81000/50 = 1620 but can't remember
why!
Sounds like you might be one of my ex-students.

The 81000 is one million divided by 12.36, the latter being the "radar mile"
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Old 21st Jan 2013, 12:29
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Q.1. When switching on the weather radar, after start-up a single very bright line appears on the screen. This means that the:

scanner is not rotating
scanning of the cathode ray tube is faulty

Q.2. On switching on the AWR a single line appears on the display. This means that:

the CRT is not scanning
the antenna is not scanning

Whats the main difference between the two?
The main difference appears to be the use of the words "very bright" in question one. If the electron gun in the CRT is repeatedly scanning either vertically or horizontally, instead of both vertically and horizontally, this will concentrate the energy in a single line. This concentration of energy is likely to make the line very bright. In this case the most accurate answer would be "scanning of the cathode ray tube is faulty".

For question 2, if the CRT is working correctly but the antenna is not scanning, then only the radar returns from the area in line with the scanner would be displayed. Whether or not these would produce a very bright line would depend upon what returns were being received from that area.

Last edited by keith williams; 21st Jan 2013 at 22:16.
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Old 22nd Jan 2013, 08:55
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Hello

Lightning Mate: Can you pls explain a little bit how to solve that question?

Keith: Thankyou!
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Old 23rd Jan 2013, 13:18
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Battery life

Hey,

I recently heard a few questions asked at some interviews but cant find the answer anywhere.

The question was, by law how long must a battery last if power is lost. I have a feeling its 30 mins but I cant be sure. I have checked EU ops and cant find the answer anywhere. Anyone know what it is for a fact?
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Old 23rd Jan 2013, 14:41
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On the Boeing 737-800 it's 60 mins in a good world... And that's using both the main battery and alternate battery!
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Old 23rd Jan 2013, 16:26
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Thanks for the reply. Do you know if that is the legal limit of one hour (on a good day) or is that Boeing's own choice to make it last longer?
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Old 24th Jan 2013, 17:17
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Ground school for CPL (H)

Hello Everyone.

I am looking for regular or part time ground school for CPL(H) Exam. Is there any in the UK and how much it cost?

Any suggestion is much appreciated.

Cheers
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Old 26th Jan 2013, 03:51
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How does one do this one:

If a radar pulse contains 300 cycles of RF energy at a frequency of 600 MHz, the physical length of the pulse is:

1550 metres
150 metres
1.5 metres
0.15 metres

thanks
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Old 26th Jan 2013, 07:05
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"Q.1. When switching on the weather radar, after start-up a single very bright line appears on the screen. This means that the:

scanner is not rotating
scanning of the cathode ray tube is faulty"

Interesting - as one who used to service monitors my first port of call would be the power supply

Deben - check out the big ad on the right
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Old 26th Jan 2013, 08:56
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If a radar pulse contains 300 cycles of RF energy at a frequency of 600 MHz, the physical length of the pulse is:

1550 metres
150 metres
1.5 metres
0.15 metres
Wavelength = c / frequency

c = 300 000 000 m/sec. f = 600 MHz = 600 000 000 cycles/sec

Wavelength = 300 000 000 / 600 000 000 = 0.5 metres

Pulse length =cycles x wavelength

Pulse length = 300 cycels x 0.5 m/cycle = 150 metres
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Old 26th Jan 2013, 11:30
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Thanks Keith

I was getting uptil 0.5 meters but could'nt figure out that Pulse length = cycles x wavelength

Can you please explain this one too:

What is the PRF given 50 micro second pulse width and a range of 30 nm:

1620 pps
810 pps
3240 pps
3086 pps

Thanks

Last edited by Haroon; 26th Jan 2013 at 11:34.
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Old 26th Jan 2013, 13:50
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To produce unambiguous range information the radar system must remain silent between pulses for sufficient time for the pulse to travel out to the furthest target and return to the antenna.

The distance traveled by the signal = c x travel time

The travel time = the time between the transmission of successive pulses, which is equal to 1/PRF.

So distance traveled by the signal = c x 1/PRF which = c / PRF

But the signal must travel out to the furthest target and back to the antenna, so maximum range to the furthest target is half of the distance that the signal travels.

So maximum range = c / ( 2 x PRF)

Rearranging this equation gives

PRF = c / ( 2 x max range)

C is the speed of light, which is approximately 300 000 KM/sec or 162 000 NM / second.


For a range of 30 Nm the above equation gives

PRF = 162 000 NM/sec / ( 2 x 30 NM) = 2700 pulses per second

So your initial calculation was correct, but 2700 pps is not an option in this question.


BUT EASA CQB 15 does contain the following similar question.



The maximum pulse repetition frequency (PRF) that can be used by a primary radar facility in order to detect targets unambiguously at a range of 50 NM is?

A 713 pps
B 610 pps
C 1620 pps
D 3240 pps

The correct answer to this question is option C 1620 pps

This can be calculated at follows

PRF = 162 000 NM/sec / ( 2 x 50 NM) = 1620 pulses per second



The 50 micro second pulse width quoted in your question does not affect the maximum range, but will determine the minimum range.

Last edited by keith williams; 26th Jan 2013 at 13:52.
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Old 26th Jan 2013, 15:12
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Thanks for the explanation Keith.

That's what I thought. However I got confused with the post:

81000/50 = 1620

81000 is one million divided by 12.36, the latter being the "radar mile"
Regards

Last edited by Haroon; 26th Jan 2013 at 15:13.
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Old 26th Jan 2013, 16:25
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I have not used the Radar Mile method before but doing a GOOGLE search revealed this.

Radar timing is usually expressed in microseconds. To relate radar timing to distances traveled by radar energy, you should know that radiated energy from a radar set travels at approximately 984 feet per microsecond. With the knowledge that a nautical mile is approximately 6,080 feet, we can figure the approximate time required for radar energy to travel one nautical mile using the following calculation:

A pulse-type radar set transmits a short burst of electromagnetic energy. Target range is determined by measuring elapsed time while the pulse travels to and returns from the target. Because two-way travel is involved, a total time of 12.36 microseconds per nautical mile will elapse between the start of the pulse from the antenna and its return to the antenna from a target.
This 12.36 microsecond time interval is sometimes referred to as a RADAR MILE, RADAR NAUTICAL MILE, or NAUTICAL RADAR MILE.

1 Radar Kilometer = 2 1000 m = 6.66 s (1)

3 108 m/s


1 Radar Mile = 2 1852 m = 12.35 s (2)

3 108 m/s

The range in kilometers to an object can be found by measuring the elapsed time during a round trip of a radar pulse and dividing this quantity by 6.66.

The range in nautical miles to an object can be found by measuring the elapsed time during a round trip of a radar pulse and dividing this quantity by 12.36.

If we use PRT = the Pulse Repetition Time

PRT = 1 000 000 micro seconds / PRF


Range = PRT / 12.36 from GOOGLE extract

PRT / Range x 12.36


For 39 NM range

30 NM x 12.36 = 370.8

PRF = 1000000 microseconds / PRT = 1000000 / 370.8 = 2696

Which is approximately 2700.

For 50 NM range

50 NM x 12.36 = 618

PRF = 1000000 microseconds / PRT = 1000000 / 618 = 1618

Which is approximately 1620
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Old 26th Jan 2013, 16:54
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Thanks Keith
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Old 26th Jan 2013, 19:41
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ATPL Met exam

Hi everyone,

I would just like some peoples thoughts on the ATPL Met subject.

I have the Oxford OAA CDs for Module 1 of the CATS course which are excellent and I'm going to be properly starting Met tomorrow. Generally, Met seems to be one of the harder subjects of the ATPL course and I'm just wondering how many weeks or even months people are taking to study for Met?

I have been studying 5 - 6 hours a day and would appreciate someone giving me an indication of just how big/complex the subject is and their experiences.

Thanks in advance
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Old 26th Jan 2013, 20:20
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Can't really comment on amount of time to spend as I went integrated and completed the whole syllabus in 2 weeks but what I will say is that it's (for me) probably the most interesting subject, try to enjoy learning it - you'll find it one of the most useful subject and most important to retain when you're flying.

Good luck!
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Old 27th Jan 2013, 14:55
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Get access to question bank, so you can test your self and see what you might get on the exam. A lot of tricky questions!
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Old 27th Jan 2013, 17:25
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Hi

The range to a required waypoint presented by RNAV system is:

plan range or slant range depending on RNAV settings
plan range
slant range
neither plan range nor slant range

Isnt it always plan range that is presented?

thanks
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