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Author Topic: Newbie wih a question  (Read 7321 times)

Offline medik06

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Newbie wih a question
« on: November 30, 2005, 05:29:10 PM »
Please excuse the questions if they sound really stupid, im new at aviation stuff and the little i know so far if from books im getting from the library reading in my spare time(Im a medical student by the way).

2 Questions...1. how come QNH seems to change after a period of time, i thought that was altitude of a specific area(say Heathrow airport), used to give all planes in the area a reference mark to measure their altitude from?? :?:

2. I have just hear a speadbird given the following instructions in 2-3 minute intervals.. "turn left 70 degrees"..."turn left 40degrees"  and "turn left 290 degrees".... the first two i can understand but how can you turn left 290degrees..... i thought you cando 180 degree(well 179) for left and also 180 for right?? :?:

sorry for the long waffle...
thanks

Medik06  :?:



Offline davolijj

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Re: Newbie wih a question
« Reply #1 on: November 30, 2005, 06:18:48 PM »
Quote from: medik06

2 Questions...1. how come QNH seems to change after a period of time, i thought that was altitude of a specific area(say Heathrow airport), used to give all planes in the area a reference mark to measure their altitude from?? :?:


I assume QNH is the altimeter setting or barometer (pressure)....this changes constantly with different weather patterns.  It's important for all aircraft to have the most current setting as their static instruments depend on it.  I believe you're thinking of field elevation which does remain constant.
Quote from: medik06

2. I have just hear a speadbird given the following instructions in 2-3 minute intervals.. "turn left 70 degrees"..."turn left 40degrees"  and "turn left 290 degrees".... the first two i can understand but how can you turn left 290degrees..... i thought you cando 180 degree(well 179) for left and also 180 for right?? :?:


These degrees are called vectors.  A left turn 40 degrees represents a turn to a heading of 040 (northeast).  A 360 heading is due north, 180 due south, 90 degrees would be east, 270 degrees west, etc.

Offline mkreilein

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Re: Newbie wih a question
« Reply #2 on: December 01, 2005, 12:50:04 AM »
That's bad phraseology if it's here in the US...unless he REALLY did want him to turn 290 deg to the left...that's some mental math that could be alleviated by giving the heading.  If you're flying 270 and he wants you to  turn 290 degrees to the left...AAAAHHHH...head spinning!

Probably supposed to be like these...

To tell an A/C to turn TO a particular heading...

"SpeedBird 114, turn right heading 090"
"SpeedBirs 114, turn left heading 350"

etc...

To tell an A/C to turn BY a specific number...like he's going on heading 360 and you want him to fly heading 020 (20 degrees more to the right)...

"United 1125, turn 20 degrees right"

Helps clear up EXACTLY what you're talking about.

See this page...it's the FAA's "Order 7110.65P" = The ATC Bible...it'll answer ALL your questions about it here in the US.

http://www.faa.gov/ATpubs/ATC/Chp5/atc0506.html#5-6-2

Offline znymz

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Newbie wih a question
« Reply #3 on: December 01, 2005, 12:50:02 PM »
My assumption on the last one was for the Speedbird to turn left to a heading of 290 degrees, though turn 290 degrees left is a valid clearance. I don't think anyone would say that unless there was a screwup somewhere and you needed the pilot to come around on a left turn a full 290 degrees.

Offline Scrapper

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Newbie wih a question
« Reply #4 on: December 01, 2005, 04:36:31 PM »
Occasionally I've told people to go more than 180 degrees, but I usually specify long way around (that's how we do it for aircraft in the navy... example: Let's say someone is going 000 (north) and you want him to go to a course of 150 but for whatever reason you want him to go left, the way I would do it is: "Speedbird 114, turn left heading 150 long way around" so there's no confusion and the pilot knows I didn't really mean right...

Is that how FAA or Nav Canada does it as well?

As for QNH davolijj is correct... it is the barometric pressure (in inches of Mercury) and changes constantly with weather... standard is 29.92, which is what aircraft use if they don't know or if they are above 18,000 feet.  Lower than that, it is very important to give the right setting and to ensure all aircraft are on the same setting... even minor changes make big differences in altitude (most aircraft use barometric altimeters, which measure your altimeter by comparing the air pressure outside with what the air pressure is at sea level... so when you say altimeter 29.75, what you're doing is saying based on the pressure at your airport, sea level pressure would be 29.75...).  Most people don't realize how important this is but a difference of one decimal point (ie. btw 29.90 and 29.80) is equivalent to about 100 feet of altitude... so you can imagine how messy that can be if you're using the wrong setting...

Offline medik06

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newbie with a quesion
« Reply #5 on: December 02, 2005, 08:27:45 PM »
awesome, thanks guys, i get it... Scrapper man, you really broke it down for me, youve got a gift man..makes`sense now..... thanks guys

Offline Scrapper

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Newbie wih a question
« Reply #6 on: December 03, 2005, 10:20:44 AM »
Tell my boss... :P