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| Thread title | Replies | Last modified |
|---|---|---|
| Meteorology for Navigation (Instruments) | 0 | 10:01, 30 March 2015 |
“Meteorology for Navigation (Instruments)” is a Talk about the application of pure meteorological theory to the subject of aircraft navigation instruments, namely: the Gyroscopic instruments, and the Pressure instruments.
Among the Gyroscopic instruments are: the DGI; Magnetic Compass; Artificial Horizon; Rate of Turn Indicator; and the INS.
Among the pressure instruments are: the Altimeter; Air Speed Indicator; Vertical Speed Indicator; and the Mach Meter.
The subject of aircraft “navigation (instruments)” is taught only in Aviation Schools, whereas pure meteorological theory is taught only in meteorological institutes. The two groups of teachers normally do not mix. So, this Talk acts as a link between them.
Contents
ABOUT THE AUTHOR[edit]
Grace, N. Lwanga holds a Bachelor of Science in Physics and Mathematics; a Diploma/ Master of Science in Meteorology; and an Airline Transport Pilot License (A.T.P.L). He has worked as a Meteorologist for 13 years and as a Ground School Navigation Instructor for 8 years. For more information, please copy this www.linkedin.com/in/gracelwanga and paste it in your URL address bar.
INTRODUCTION[edit]
Welcome to this Talk. There are two basic objectives in presenting this Talk: the first is to help the meteorologist acquaint himself with the new field of air navigation; the second is to solve the navigation (instruments) questions by means of an objective method.
The reader may expect to gain three benefits from this Talk: first, a consistent theoretical background to the dead- reckoning of the air-navigation instruments; second, an encouragement for doing further applications of dynamical meteorology to air navigation and third, a simplified method of defining the basic navigational terms.
The following levels of academic training will enable the reader to follow most of the material in this Talk: a Commercial Pilot’s License (C.P.L), or an Air Training Pilot’s License (A.T.P.L), or a Class One meteorological training course, or a High School course in vector calculus.
A reader who wishes to sit for a license examination should not exclusively use the mathematical symbols found in this Talk without seeking the opinion of the examiner.
PART 1[edit]
Among the flight instruments of an aircraft, there are two categories: the engine instruments, and the navigation instruments. Engine instruments are used for the aircraft engines, while navigation instruments are used for navigating the aircraft.
Navigation instruments are further divided into two groups: the radio instruments and the dead- reckon (DR) instruments. Radio instruments work by ground radio aids, while DR instruments work without any ground aids. Radio instruments are explained by radio wave theory, while various branches of science explain DR instruments. One of the branches is meteorology.
For a long time, aviation meteorology has been regarded as the only branch of meteorology relevant to flight information. Meteorologists know aviation meteorology as synoptic meteorology; and it is restricted to 'en- route' weather reports. The other (and less familiar) type of meteorology for flight information is meteorology for navigation (instruments).
Meteorologists know meteorology for navigation (instruments) as dynamical meteorology. They use it for the Numerical Weather Prediction (NWP) models. In aviation, however, it is used for the DR navigational instruments. The sub- branch instruments is intended to contrast the branch from the aviation meteorology, which is already familiar to us.
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Navigation may be defined as the free movement of a vehicle, from one station to another, for the fulfillment of a purpose. Such a movement may be carried out on the sea, land or in the air; aided or unaided by external navigational equipment; and finished off in the most economical, punctual and safe plan.
In aviation, when a vehicle is navigated without the aid of external navigational facilities (such as a radar or landmarks) the navigation is called Dead Reckoning (DR) navigation. DR navigation is a remarkable success for Man, because he has no flying instinct. For example, the birds cannot fly without depending on the outside visual contact; although they possess an instinct and a very long flying experience.
[edit]
Meteorologists will agree that before making a weather report (or a forecast) in an area, a set of meteorological data for the area must be collected, plotted and then analyzed on a chart. The data is measured by special meteorological instruments, which are launched into the sky or located on the ground. The same kind of procedure may be compared for DR navigation.
Drg. 1.1. Labels used for a triangle- of- velocities. According to trigonometry, if two sides of a triangle are known, then the third side can always be calculated. The triangle of velocities is used for the leg between take- off and approach. Its labels are defined as follows: CB is the mean wind forecast for the pressure surface p, period t, and ground position B(x,y); AC is the aircraft Heading and True Air Speed; AB is the aircraft ground speed and track; position A(xo,yo) is the departure; B is the DR ground position; C is the air position; and is the wind-drift correction for the aircraft flight. (Courtesy of Soroti Flying School, Uganda).
In Fig. 1.3 and 1.5, some of the DR navigation instruments are shown on the aircraft flight panel. Monitoring of the readings gives an analysis of the aircraft flight. Usually such analysis is made on a navigation chart by the flight navigator; and its principle is to solve the triangle of velocities (Drg.1.1) relevant for the flight leg.
It is a routine exercise to solve the triangle of velocities. So, the navigator uses special kits like the navigation computer (Fig. 1.2) as a facility.
Fig. 1.1. Side “B” of the navigation computer. The photograph shows the B- side of the navigation computer. The B- side can be used for computing the True Air Speed; Rectified Air Speed; Mach Number; Density Altitude; True Altitude; time gone; fuel and distance conversions; multiplication and division. (Courtesy of Soroti Flying School, Uganda).
Track and ground speed is the velocity, which is most often required in the DR navigation. It is the result of both the wind and the heading-and-True Air Speed vector. Therefore, the accuracy of the DR navigation depends not only on the wind forecast but also the accuracy of the reference instruments.
Many ICAO annexes stipulate the standard of accuracy expected of the wind reports or forecasts, as well as the accuracy expected of the serviceable instruments. The background theory for the errors of these instruments, however, is not stipulated.
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According to section 1.1, if the restrictions of fuel economy; punctuality; and safety are removed from the definition of navigation, then the triangle of velocities (Drg. 1.1) can be used to explain the principles of DR navigation. Further, if the wind report is assumed to be correct, then the errors affecting the theory of the DR navigation are based on the instruments. Therefore, a study of the DR navigation reduces itself to merely a study of the DR navigation instruments.
Like all basic skills, the DR navigation started from a simple set of instruments such as the Altimeter; Air Speed Indicator; and the Magnetic Compass. Eventually it expanded to include the Vertical Speed Indicator; the Directional Gyro Indicator; the Rate of Turn Indicator and the Artificial Horizon (Fig. 1.3 and 1.5). In a few aircrafts, the DR navigation instruments also include the Inertial Navigation System and the Mach meter.
All these instruments are divisible into two groups: the pressure instruments, which work by means of the air capsules (see part 2); and the gyroscopic instruments, which work by means of rotors (see part 2). In section 2.2 of part 2, the two groups of instruments are further distinguished according to their dynamical responses during the aircraft maneuvers.
Observations show that the pressure instruments (i.e. The Speed Indicator; the Vertical Speed Indicator (and the Mach meter) misread during the aircraft maneuvers, whereas the gyros do not.
In 1970, professor Holton of the University of Washington, U.S.A., showed that the wind could be decomposed into the sum of ‘rotational’ and 'irrotational non-divergent’ vectors [see part 2]. This work can be extended to cover the theory of the DR navigational instruments, whereby the gyroscopic instruments correspond to the rotational motion; and the pressure instruments correspond to the irrotational non-divergent motion of the aircraft.
Fig. 1.2. “Side A” of the navigation computer. The photograph shows the A-side of a navigation computer. The A- side can be used for finding the True heading; Track; ground speed and the wind. The letter L (shown at the top right hand corner of the wind arm) stands for all air speeds, which are below 300 knots True Air Speed. The flip side shows the letter H, which denotes the air speeds above 300 knots True Air Speed. (Courtesy of Soroti Flying School, Uganda).
A good example of the rotational motion is the absolute vorticity. This term is quite familiar in meteorology, and it is constructed by performing the curl operation onto the basic equations of motion (see part 2). The convective and advective atmospheric processes best illustrate the irrotational non-divergent motion, and it is generally obtained by the elimination of rotational factors, as shown in part 2.
Fig. 1.3. Cessna 172. The photograph shows the instrument panel of a Cessna 172, depicting the DR navigation instruments. The top row shows the Air Speed Indicator (second from the left); Artificial Horizon; and Altimeter. The middle row shows the Rate of Turn Indicator (second from the left); Directional Gyro Indicator; and Vertical Speed Indicator. The Mach meter and the Inertial Navigation System are not used on this type of aircraft. (Courtesy of Soroti Flying School, Uganda).
Fig. 1.4. Direct Reading Compass. The photograph shows the instrument panel of a Cessna 172, depicting the direct reading compass, which is called the E-type. It is located in the fore-aft plane of the aircraft, and all bearings are obtained in conjunction with the compass deviation card. (Courtesy of Soroti Flying School, Uganda).
