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Airspeeds- Basic Principle of Operation

Main Components

The major components of an air speed indicator are:

Case
Diaphragm
Dial
Pointer
Mechanical Linkage
Hair Spring
Jewels and Pivots

Case
A standard air speed indicator for general aviation comes in a 3^1/8inch diameter case. This is a standard size for most general aviation indicators. It is important that an air speed case be air tight as the case contains the static pressure input.

A leaky air speed case will cause the indicator to give erroneous readings. There are two input ports on the back of the case. These ports are the Pitot port and the Static port. These ports are connected to aircraft Pitot tube and Static port respectively.

Diaphragm
A diaphragm is essentially a balloon made of very thin metal. Typical metals used for this purpose are copper or brass. The diaphragm is sealed at all points except for one very thin pressure input tube. This tube is located directly in the center of the back face of the diaphragm. The pressure tube is connected directly to the Pitot port. The thickness of the metal used in manufacturing the diaphragm will determine the range of the air speed indicator (thin = low speed, thick = high speed). It is important to note that this diaphragm is extremely delicate. Picking up a unit a blowing air into the input port will damage the diaphragm.

Dial
The dial will contain the speed information and any pertinent range marks.

Pointer
The pointer points to the current speed of the aircraft as indicated on the dial.

Mechanical Linkage
The mechanical linkage connects the pointer to the diaphragm.

Hair Spring
The hair spring returns the pointer to zero when pressure is removed from the indicator.

Jewels and Pivots
The pivot is the spindle or axle for the pointer. Jewels are glass bearings on which the pivot rotates.

Understanding Your Airspeed

General Information

In its purest form, an air speed indicator is simply a differential pressure gauge. This means that it is displaying the difference between two different pressure sources that are being simultaneously applied. In the case of airspeed, the two pressures are Pitot Pressure and Static Pressure.

Pitot Pressure is the pressure that is generated when air is forced into the aircraft Pitot tube because of the forward motion of the aircraft. The Pitot tube is a slender tube that is typically mounted near the front of the aircraft. The opening of the tube is facing forward so that air is forced into it. This tube is connected directly to the input of the air speed indicator.

Static pressure is the standard air pressure at the current altitude. Static pressure will vary due to altitude changes and due to changes in weather.

Standard Static pressure at sea level altitude on a “standard” day is typically 29.92 In Hg (inches of mercury) as measured on a class “A” barometer. Static pressure on an aircraft is measured at the static port. The static port is a small hole usually located on the side of the aircraft. This pressure is applied to all instruments in the aircraft static system, of which the air speed indicator is one.

Airspeed Indicators – Product Familiarization

General Information

The Air Speed indicator provides the basic function of indicating to the pilot the current speed of his aircraft. The indicator is normally a 3^1/8”size dial face with a single pointer. Location of this indicator is typically in the top row of instruments on the left side of the panel. In a standard “T”, configuration instrument panel the airspeed indicator would be located just to the left of the attitude gyroscope.

Types of Air Speed Indicators

Indicated Air Speed Indicator
True Air Speed Indicator (Manually Operated)
True Air Speed Indicator (Automatically Operated)
Maximum Allowable Air Speed Indicator
Mach Air Speed Indicator
F1 Air Speed Indicator
Helicopter Air Speed Indicator
Dual Scale Indicators

Indicated Air Speed Indicators
The Indicated Air Speed (IAS) Indicator provides the pilot with a speed-reading, which is based only on Pitot and Static pressure inputs. This type of indicator does not take into account other variable inputs such as temperature or altitude.

True Air Speed Indicator (Manually Operated)
The manually operated True Air Speed Indicator provides the pilot with a method of inputting pressure altitude and outside air temperature via a knob on the instrument face. This knob controls a sub-dial which will provide the pilot with “True Airspeed’ data

True Air Speed Indicator (Automatically Operated)
An automatically operated True Air Speed Indicator has both a temperature bulb and altitude aneroid built into the instrument. This type of indicator will always display the true speed of the aircraft without user input.

Maximum Allowable Air Speed Indicator
A Maximum Allowable Air Speed Indicator has a second pointer that is set to indicate the maximum permitted speed of the aircraft at the current altitude. This pointer is usually a red barber pole.

Mach Air Speed Indicator
A Mach Air Speed Indicator indicates the speed of the aircraft as a percent of the speed of sound. When an object is traveling at the speed of sound it is traveling at Mach 1, twice the speed of sound would be Mach 2, etc…

F1 Air Speed Indicator
An F1 Air Speed Indicator is a high-speed indicator, typically 600 Kts. or higher and typically is a drum type indicator.

Helicopter Air Speed Indicator
A Helicopter Air Speed Indicator is typically an Indicated Air Speed Indicator that has very low speed-readings.

Dual Scale Indicator
One Knot, one Nautical Mile, is equal to 1.15077945 Statute Miles.
One Statute Mile is equal to 0.86897624 Knots. A dual scale air speed indicator will indicate both Knots (KPH or Kts.) and Statute Miles (MPH). Always verify if the customer wants the Kts. or MPH on the outer or inner scale of the dial.

TGH Aviation Awarded Military Contract

TGH Aviation Awarded Military Contract for Support of the United States Air Force

Auburn, CA, December, 8^th, 2015

TGH Aviation, based out of the Auburn Airport in Auburn CA, is proud to announce it has been awarded a multi-million dollar five year contract by the U.S. Air Force for the purpose of overhauling all Fuel Flow Transmitters, 8TJ61 Series, for the T-38 Supersonic Jet Trainers. The T-38 is the primary training aircraft for the Air Force. More than 60,000 USAF pilots have trained in the T-38 since it entered service in 1961, when it was the world’s first supersonic trainer. USAF T-38 trainers are primarily used by the Air Education and Training Command for joint specialized undergraduate pilot training (JSUPT), but the aircraft are also used by the Air Combat Command for its companion training program and by the Air Force Materiel Command to test experimental equipment.

“The award of this contract extension is an affirmation by the Air Force of the extraordinary service and quality of workmanship provided by the employees of TGH Aviation,” said TGH President Richard Anderson regarding the recent contract award. For more than 56 years TGH Aviation, formerly The Gyro House, has been an industry leader in flight system support. The company exudes confidence in its ability to provide best-in-class repair, overhaul and new products for a wide range of aviation needs, with the fuel flow support being at the heart of them. TGH operates four full time Fuel Flow Transmitter calibration laboratories. All tools and test equipment utilized by TGH, for the “Return to Service” of Fuel Flow Transmitters, are calibrated, certified and traceable to the National Institute of Standards and Technology (NIST). TGH uses only quality parts that are OEM or FAA approved in the completion of repair work on Fuel Flow Transmitters. The flowmeter operates on the principle that the rotation the fuel by the constant-speed inlet impeller tends to rotate the stationary outlet turbine in an amount which is proportional to the rate of fluid flow through the device measured in pounds per hour. A constant speed motor turns at 8,000 RPM, driving the inlet impeller at 206 RPM through a reduction gear. The inlet struts and inlet impeller combine to impart a clockwise rotation to the incoming fluid, which tends to turn the outlet turbine. The outlet turbine does not rotate through a complete revolution, but as the fuel passes from the inlet impeller with a clockwise spin, it causes the outlet turbine to deflect from its neutral position against the action of a spring. The angular deflection is directly proportional to the mass rate of fuel flow. The outlet turbine deflection is sensed by a synchro transmitter which is directly coupled to it, and provides a signal proportional to the rate of flow to an indicator.

TGH Aviation has a well-known reputation for their precision and innovation within the aviation industry. In addition to military aircraft repair, TGH also overhauls fuel flow transmitters for high performance aviation aircraft, corporate/commuter aircraft and large commercial aircraft. For more information on TGH aviation and their capabilities please check out their website at www.tghaviation.com.

Frequently Asked Altimeter Questions

The pointers on my altimeter are very jumpy and stick sometimes.
What is wrong?

The altimeter is exposed to the outside atmosphere. This includes all of the dirt and dust present in that atmosphere. Dirt and dust will get into the gears and cause them to stick and bind, the vibration from the aircraft will help the gears to overcome this problem but they will be very jumpy and become worse as the unit gets older. This unit needs an overhaul.

Can I convert my milli-bar altimeter to InHg or vice versa?

Yes, altimeter dials can be converted, provided that the manufacturer has published a procedure for doing so. If there are no published, FAA Approved, procedures then, no, this modification cannot be accomplished.

How often does my altimeter need to be calibrated?

The FAA requires that the aircraft static system be tested and certified biannually. The altimeter is a part of that system.

The altimeter ID plate says that it is a 35,000 Ft. altimeter, but the repair shop that overhauled my altimeter marked the unit as being certified to 30,000 Ft. Why the difference?

As altimeters get older and the parts wear the unit will become inaccurate at the higher end of its range. This does not mean that the altimeter can no longer be used; it just has to be used at the lower altitude. Therefore, it will be sold only to customers who request a lower altitude unit, typically general aviation.

What is a “car” altimeter?

Sometimes, if a unit is so old and worn that it can no longer be used in an aircraft then these units are sold at a discounted price for use in other than aircraft, typically people will use them in motor homes, cars, and boats.

Along with my altimeter, I received a correction card. What is that?

Altitude is a non-linear function. It is impossible to calibrate an altimeter to be absolutely accurate at all altitudes. Therefore, a certain amount of error is allowable. The correction card advises the users of the amount of error in a particular altimeter. As each altimeter will have its own characteristics, the error card is identified with the unit’s serial number.

What is a TSO?

TSO stands for Technical Standard Order. This is an FAA document, which defines how a specific type of instrument should work in order to be considered airworthy. Altimeters manufactured by the following companies typically are manufactured to TSO standards: United Instruments, Kollsman, Garwin, and Aerosonic. Altimeters manufactured by the following companies typically are not qualified to TSO specifications:UMA & Falcon. If an instrument is certified to a TSO, it must state such on the ID plate. If it does not state the TSO on the nameplate then you must assume that it is not qualified to the TSO.

What are the typical failure modes of an altimeter?

Sticky/jumpy pointers
Inability to properly adjust the Kollsman window
Out of calibration
Worn pivots and/or jewels
“Oil-canning” of the aneroid

How to Read a 3 Pointer Altimeter

A three-pointer altimeter, as its name implies, has three different pointers on the front dial. They are the 100-foot pointer, the 1000-foot pointer, and the 10,000-foot pointer. The medium length pointer is the 100-foot pointer, the shortest pointer is the 1000-foot pointer, and the longest pointer is the 10,000-foot pointer. The altimeter dial has 10 major indices numbered 0 through 9. In between each major indice are 4 minor indices. The value of these indices is dependent on the pointer being read. When reading the 100-foot pointer each minor indice equals 20 feet, each major indice equals 100 feet. When reading the 1000-foot pointer each minor indice is equal to 200 feet, each major indice is equal to 1000 feet. When reading the 10,000-foot pointer each minor indice is equal to 2000 feet, each major indice is equal to 10,000 feet. The altimeter in figure 1 is indicating 11,520 feet and is read as follows:

The 10,000 foot pointer is past the 1 and not yet up to the

2 and so it is read as: 1 x 10,000 = 10,000 +

The 1,000 foot pointer is past the 1 and not yet up to the

2 and so it is read as: 1 x 1,000 = 1,000 +

The 100-foot pointer is 1 minor indice past the 5 and so

Therefore, it is read as: 5.2 x 100 = 520

The indicated altitude is the sum of the pointers: 11,520

Figure 1: Three-Pointer Altimeter

Getting to know your Kollsman Window and Barber Pole

Reading the Kollsman Window ( Barometric Reading)

The Kollsman window is located at the 3 o’clock position on the altimeter dial. This window allows access to read a sub-dial, which contains the barometric readings. The arrowhead indice located precisely at the 3 o’clock position on the altimeter’s main dial is used as the reference point for reading the barometric sub-dial. Most altimeters will have a sub-dial, which covers the readings from 28.1 InHg (inches of mercury) to 31.0 InHg. On the sub-dial each major indice is read as 0.1 InHg, each minor indice is read as 0.02 InHg.

The Altimeter in Figure 1 reads 29.92 InHg

As previously, stated, weather conditions will greatly affect the pressure of the atmosphere (the barometric reading). Altimeters report altitude as a function of atmospheric pressure. Typically pilots will obtain a local barometric reading from the nearest airport. They will then set the Kollsman window to the setting that they received. This action will adjust the altimeter reading, eliminating error due to local weather conditions.

Some altimeters will have a Kollsman Window, which reads out in milli-bars in lieu of InHg. These are usually altimeters designated for use in Europe. However use of milli-bars has become more common in the U.S. during the last few years. Milli-Bars is just another unit of measurement, 1013.2 milli-bars = 29.92 InHg.

The Barber Pole

The “Barber Pole” on the face of the altimeter is visible only when the altitude is above sea level. When the altitude is below sea level the barber pole is no longer visible. This is provided to avoid the error of reading –1,000 Ft. as being +10,000 Ft.

Altimeters Major Components

The major components of the altimeter are:

Case
Aneroid and Mechanical Linkage Assembly
Dial and Pointer Assembly
Barometric Dial and Setting Assembly

Case

A standard altimeter for general aviation comes in a 3^1/8”diameter case. This is a standard size case for most general aviation indicators. It is important that the altimeter case be airtight as the case contains the static pressure input. A leaky case will cause the indicator to give erroneous readings. There is one pressure input on the back of the case. This input is the “STATIC PRESSURE” input and is connected directly to the static port on the aircraft, which is exposed to the outside atmosphere.

Aneroid and Mechanical Linkage Assembly

An aneroid is essentially a balloon made of very thin metal. Typical metals used for this purpose are copper or brass. The aneroid, when first manufactured is sealed at precisely 29.92 InHg that is the standard atmospheric pressure for a standard day at sea level. Because the aneroid is sealed at this specific pressure, any change in the pressure surrounding it will cause it to either expand or contract in a manner which is directly proportional to the change in the surrounding pressure. This expansion and contraction are relayed to the pointer via the mechanical linkage assembly. It is important to note that the aneroid is extremely delicate. Very fast changes in altitude can and will damage the aneroid.

The Mechanical Linkage Assembly is comprised of a link, several types of gears, glass jewels, pivots, a hairspring and a bimetal assembly.

The link directly connects the aneroid to the gears. The gears transfer the aneroid movement to the pointer. The glass jewels are bushings for the gear pivots to ride in. Pivots are the axles for the gears. The hairspring assembly provides an anti backlash function for the gears helping to eliminate hysteresis errors. The bimetal assembly provides the function of correcting for temperature changes.

Dial and Pointer Assembly

The dial and pointer assembly contains the read out information for the pilot. A typical three-pointer altimeter has, as the name would imply, three pointers. These are connected to the aneroid via the gears. There is also a main dial with major indices numbered 0 thru 9. The 0 indice is located at the 12 o’clock position on the dial. The numbers 1 thru 9 are then linearly distributed around the dial face. Each main indice is subdivided by 4 minor indices, which are equally spaced.

Barometric Dial and Setting Assembly

Behind the main dial is a sub-dial. The sub-dial is viewable at the 3 o’clock position on the main dial. This opening is called the “Kollsman Window”. The reading in the Kollsman Window is settable via the barometric adjust knob which is located, on modern altimeters, in the 7 o’clock position of the instrument face.

Older altimeters will have the barometric adjust knob located at the 6 o’clock position.

Understanding Altimeters

In its purest form, an altimeter is simply an absolute pressure gauge. This means that it is displaying the pressure being exerted by the atmosphere at its current location.

The earth is surrounded by an atmosphere. This atmosphere is the air that we breathe. The atmosphere is held in place by the earth’s gravity. The atmosphere has a specific weight. The weight of the atmosphere is approximately 14 pounds of weight for every square inch of earth when measured at sea level on an average day.

An accurate method of measuring this weight is to use a barometer. A barometer is a reservoir filled with mercury. The reservoir has two openings; one opening is exposed to the atmosphere and the other empties into a glass tube. The atmosphere pushes down on the mercury within the reservoir causing the mercury to fill up the glass tube. How far the mercury goes up into the glass tube is directly proportional to the weight of the atmosphere pushing it. This is why barometric pressure is normally expressed in terms of “Inches of Mercury (InHg)”.

At sea level, on an average day, the barometric pressure is 29.92 InHg. However, this will vary constantly depending on the weather. Stormy weather tends to pull the atmosphere away from the earth’s surface causing lower pressure. Hot, dry weather pushes the atmosphere down causing higher pressure.

The weight of the atmosphere also changes depending on altitude. The closer to sea level that you are, the more air there will be, consequently the atmosphere will weigh more. As you go higher in altitude, the less dense the atmosphere will be, therefore less weight or pressure is exerted. An altimeter measures this change in atmospheric weight as expressed in terms of pressure or feet of altitude.

All About Altimeters

General Information

The altimeter provides the basic function of indicating to the pilot the altitude of the aircraft above mean (average) sea level (MSL). The indicator is normally a 3^1/8” size dial face with multiple pointers or a combination of pointers and counter drum. Location of this indicator is typically in the top row of instruments near the center of the instrument panel. In a standard “T” configuration panel the altimeter is just to the right of the attitude gyroscope.

Types of Altimeters

The various types of altimeters include:

Three pointer altimeter
Counter Drum Altimeter
Encoding Altimeter

Typical altimeter ranges are:

–1000 Ft. to +20,000 Ft.
–1000 Ft. to +35,000 Ft.
–1000 Ft. to +50,000 Ft.
–1000 Ft. to +80,000 Ft.

Most general aviation altimeters will fall into the first two ranges. Ranges above 35,000 Ft. are typically corporate jets, commercial aircraft, and military aircraft.

Three Pointer Altimeter

The three-pointer altimeter is the most common type of instrument used in general aviation. It is named as such because it utilizes three pointers in order to display the current altitude. One pointer is used to display 100 Ft. increments. A second pointer is used to display 1000 Ft. increments and the third pointer displays 10,000 Ft. increments. The Technical Information Section of this document provides instructions on how to properly read a three-pointer altimeter.

Counter Drum Altimeter

The counter-drum altimeter is named as such because it displays altitude utilizing a single pointer and a rotating drum that displays digits. The drum displays ten thousand and one thousand foot increments. The pointer displays from 0 to 999 feet.

Encoding Altimeter

An encoding altimeter can be of either the three-pointer or counter drum type of altimeter with an encoding module built into it. The encoding module takes the altitude information and converts that data into a digital code. This code is then sent via a set of wires to the aircraft transponder. A transponder is a radio device that reports the aircraft altitude to ground control radar.

Blind Encoder

The blind encoder is a very special type of altimeter. This unit has no dial or read out that is visible to the user. It has only an electronic output to the aircraft transponder. The use of a standard altimeter in conjunction with a blind encoder is often more economical than purchasing an encoding altimeter.