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ANSWERED on Thu 7 Aug 2008 - 3:42 am UTC by byrd

Question: course correction in airplanes

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barrys 

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 6 Aug 2008 09:58 UTCWed 6 Aug 2008 - 9:58 am UTC 

I have read that 75% of the time airplane pilots must do a course correction. So that in reality, the plane is only on course 25% of the time.

I would like to know what this actualy reffers to, whether is means when the plane is on autopilot or in general. I would also like a bit more information as to what systems are involved in navagating a plane, how a pilot corrects his course, how he knows to correct his course....

 

byrd 

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 7 Aug 2008 03:42 UTCThu 7 Aug 2008 - 3:42 am UTC 

Hi Barrys,

I'm not sure where you found those statistics, or in what context, but I can assure you that the conclusion that aircraft are on course only 25% of the time is really not a correct assumption. Let me try to explain. Just so you know, I am a trained, FAA licensed, instrument-rated, Commercial Pilot, so I have some knowledge and experience in this area and what I don't know, I do know where to go to find it.

Aircraft navigation is a very complex endeavor, involving many multiple factors that all have to be balanced against one another. You can't simply point an airplane in the direction you want it to go and expect it to fly straight there without guidance. That guidance involves many changes to the course, or "course correction," over the duration of any flight. There are entire books written about the subject of air navigation, and the price of this question doesn't really allow for writing another one, but I'll try to give you a brief overview, along with links to some resources you can use if you like to further your understanding.

An airplane's course simply means the direction it is flying, the path it is taking through the air. There are various factors that can affect an airplane's course, that make adjusting, or correcting it necessary. These can include wind, pilot technique, a flight plan that requires various turns and changes in direction, changes in instrument indications that allow the plane to get off course and need to be brought back to the correct path. An important part of flying is ensuring that the aircraft stays on course, and that requires many corrections throughout the duration of the flight.

Think of it like this: when you drive a car down the road, you don't just point it and let it go, do you? Rather, you plan a certain route, or decide to drive on a particular road, and then keep your hands on the wheel, constantly correcting the direction the car goes, steering it where you want it to go, rather than letting it just do its own thing. Sometimes, like on a winding, twisty mountain road, that takes more doing, while other times, say heading down a miles-long straight stretch of highway with no obstacles or other traffic, you can relax and almost let the car drive itself.

Well, it's the same way with airplanes. When you read that an airplane's course is being corrected 75% of the time, that actually mean the airplane is being kept ON course, not being flown off it. The 25% of the time there are no course corrections being made is just the time there are no factors affecting the airplane's flight path, such as during a long enroute segment of a flight at the same altitude and with no wind, sort of like that long stretch of highway.

So to answer your specific questions. I've taken the last one first, as that really is where it all begins.

1) First of all, how a pilot knows a plane is on course has to do with where he wants the plane to be going, and that starts with planning a flight and sometimes changing that plan while in flight. A pilot decides to fly from Point A to Point B. But he doesn't just take off and fly directly there in most cases. He uses several different methods to decide where to actually fly the airplane.

You might be surprised to know there are as many regulations about where you can fly an airplane as there are rules about where you can drive your car, actually even more since planes go up and down as well as fly on the level. There are maps (called charts) to follow, and airplanes must also take into consideration the terrain of the ground over which they'll be flying. For example, if there is a 15,000 foot mountain in the middle of the way the pilot wants to go, and the airplane he plans to fly can only climb as high as 12,000 feet, then he will have to plan a path around the mountain instead of over it.

And then there are rules about different kinds of airspace and places planes can and can't go. A pilot can't just fly over the middle of a big city, for instance. He has to permission from Air Traffic Control (ATC) even to fly near or inside the airspace surrounding a large city and then has to go where the controllers tell him to, all of which involves adjusting and correcting his course.

There are also invisible roads (called airways or jetways) in the sky that pilots on a certain type of flight plan needs to follow. There are intersections between roads, and sometimes a flight plan will take an airplane along one airway to an intersection, then change direction and fly along another airway, and so forth.

In addition, there are other invisible electronic signals of various kinds that also function as invisible pathways in the air, and a pilot may choose or be required to follow one or more of these in order to get where he wants to go. One example is a type radio transmitter called a VOR (stands for VHF Omnidirectional Range) that sends lines of radio signals in a circle extending out from the transmitter on the ground. An instrument in the airplane can tune in to these signals and either fly directly along them, or use them as guidance to fly a different course.

A pilot is constantly monitoring the position of the airplane by comparing it to his flight plan, or instructions from ATC, using a combination of charts, instrument readings, comparisons between instruments, and various other techniques, including the proverbial eyeball out the window. He must also monitor the condition of the instruments, comparing them against each other, to be sure they are giving accurate information so he can stay on course.

One other element that matters more to pilots than drivers on the ground is weather. A pilot certainly knows he has to correct his course if he sees a big ole thunderstorm ahead of him on his route. Thunderclouds, called cumulonimbus clouds, are extremely dangerous and turbulent, and can grow to astonishing heights reaching up into the stratosphere, in a very short period of time. Pilots are taught that cumulonimbus means "cumulo-concrete" and not to attempt to fly through these monsters. So a thunderstorm a-building means route changing. Other weather events, such as a layer of freezing rain, or the presence of a microburst, can also require a pilot to have to change course enroute.

2) How a pilot corrects his course is determined by the flight plan he is following, the type of flight it is (whether being flown by visual rules or instrument rules) and the types of equipment available in his aircraft.

But whether done by the pilot himself, or the autopilot, actual corrections are made by using the aircraft controls to turn left or right, or go up or down, or both.

3) The systems involved in navigating an airplane range from the pilot's eyeballs to a full glass cockpit in a technologically advanced aircraft so automated that it can practically fly itself and the pilot need only monitor all the gadgets and gizmos.  Instruments range from a regular old magnetic compass, just about the same thing used by hunters and boy scouts, up to high end GPS and onboard radar systems.

The two simplest ways of navigating are based on visual rules and  are called pilotage and dead reckoning. Pilotage involves finding your way by identifying checkpoints on the ground and comparing their location on a map or chart. Dead (really should be Ded for "deductive" but "dead" is how it's usually spelled) reckoning is a method that uses course, heading, airspeed and elapsed time to deduce where you are on a map starting from a known point.

Use of instruments to navigate ranges from comparing a compass heading to the planned course on a chart, to making course adjustments based on instructions from an ATC controller, to following guidance from a GPS or radar system.

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I've gathered some resources for you that will expand upon this brief explanation, and allow you to pursue it as little or as much as you like. They're listed alphabetically so do check them all out, as each has something a little different to offer that can help you understand different aspects of this very complicated subject. These are very authoritative sources with excellent information, well presented.

AOPA (Aircraft Owners and Pilots Association) is a terrific resource for learning about various aspects of aviation, including navigation. Their navigation section not only explains the principles of air navigation but has some great photos and descriptions of the various instruments used in electronic navigation. Check it out here:
http://www.gaservingamerica.org/how_work/work_navigation.htm
Here's a link to similar page, also by AOPA:
http://flighttraining.aopa.org/learntofly/overview/navigation.cfm

If you feel up to some heavy duty reading, here's the FAA's Aeronautical
Information Manual (AIM), Official Guide to Basic Flight Information and
ATC Procedures, Chapter 1, Air Navigation.
  Chapter 1, Section 1. Navigation Aids
http://www.faa.gov/airports_airtraffic/air_traffic/publications/atpubs/aim/Chap1/aim0101.html
  Chapter 1, Section 2. Area Navigation (RNAV) and Required Navigation
Performance (RNP):
http://www.faa.gov/airports_airtraffic/air_traffic/publications/atpubs/aim/Chap1/aim0102.html

There is a Free Online Private Pilot Ground School that has some excellent
instruction on basic aircraft navigation, with a link to the FAA test
questions so you can see how well you did.
  - What does a private pilot need to know about airplane navigation?
http://www.free-online-private-pilot-ground-school.com/airplane-navigation.html
  - Navigation Basics: Fundamental Concepts in Aeronautical Navigation
http://www.free-online-private-pilot-ground-school.com/navigation-basics.html
  - Aeronautic Navigation Instruments: An Overview for the VFR Pilot
http://www.free-online-private-pilot-ground-school.com/aeronautic-navigation-instruments.html
  - Practical Navigation Principles
http://www.free-online-private-pilot-ground-school.com/navigation-principles.html

Here is an excellent resource from the National Aviation and Space Administration (NASA) on basic Aviation Navigation. It has an interesting and complete tutorial on the subject, and even lets you take a quiz and print out a certificate of your achievement if you decide to follow it all the way through. It "will introduce you to how to use a compass, read an aeronautical chart, and plot a course. You will learn how to calculate flight distance, flight time and fuel consumption. You will gain an understanding of ground track and true course and how the wind affects your flight. This will provide you with a little bit of the basic knowledge that all pilots and flight controllers need for their work."
http://virtualskies.arc.nasa.gov/main/mainnavigation.html

Pilotfriend.com has a brief primer on aircraft navigation with good explanations of both pilotage and dead reckoning, as well as links to further information.
http://www.pilotfriend.com/training/flight_training/nav/basic.htm

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I hope you will find the information provided helpful. If anything isn't clear or you need further assistance, please ask for a clarification and I'll be happy to respond. Good luck, and thanks for the chance to talk aviation, which I always enjoy!

Best wishes,
Byrd

 

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