## Pitot Static System…Airspeed Calculation

Pitot Static System…Airspeed Calculation

A.  Airspeed Calculation:

Airspeed is calculated as a function of the difference between Pitot Pressure and Static Pressure as follows:

Calculated or Indicated airspeed is indicated airspeed corrected for instrument errors, position error (due to incorrect pressure at the static port) and installation errors.

Calibrated airspeed values less than the speed of sound at standard sea level (661.4788 knots) are calculated as follows:

minus position and installation error correction.

Where

is the calibrated airspeed,

is the impact pressure (inches Hg) sensed by the pitot tube,

is 29.92126 inches Hg; static air pressure at standard sea level,

is 661.4788 knots:, speed of sound at standard sea level

Units other than knots and inches of mercury can be used, if used consistently.

This expression is based on the form of Bernoulli’s equation applicable to a perfect, incompressible gas. The values forand   are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated.

Keep in mind that this is for your basic vanilla airspeed indicator and does not include calculations for TRUE Airspeed for which you must include the variables of True Temperature and True Altitude.

Stay tuned for upcoming Blogs

## Pitot Static System…. Inside & Out

Pitot Static System ….Inside & Out

A. Pitot Pressure:
Pronounced: PEE-TOE, it is a French word

Pitot pressure is the measurement of the air forced into the Pitot Tube by the movement of the aircraft through the air. Pitot tubes are mounted on the aircraft facing forward so that air is forced into them. Most small aircraft have only one tube, larger aircraft have a redundant system and will have two tubes. The most common manufacturer of these tubes is Rosemont Corp. which is a division of BF Goodrich. Also on larger aircraft, those that fly at higher altitudes, the Pitot Tube is heated in order to prevent icing, smaller aircraft typically do not have this function.

The Pitot Tube is connected directly to the back of the airspeed indicator (the Pitot input) and if the aircraft is so equipped also to the Air Data Computer via a hose which is typically either plastic or rubber

B. Static Pressure:

Static pressure is the measurement of the ambient barometric pressure at the aircraft’s CURRENT location AND CURRENT Altitude.
The Static Port is located in a position on the aircraft that will not be affected by air flow as the aircraft moves through the air. This is typically on the side of the fuselage but can also be on the back side of the Pitot Tube or any other number of locations, it varies by the aircraft. Again smaller aircraft will typically have one Static Port, larger aircraft with redundant systems will have two.

The Static Port is connected directly to the following equipment, depending on aircraft configuration: The Airspeed Indicator (Static Input), the Altimeter, the Vertical Speed Indicator, the Altitude Encoder, the Air Data Computer. Again connection is typically made via a hose either rubber or plastic.

C.  Airspeed Calculation:

Airspeed is calculated as a function of the difference between Pitot Pressure and Static Pressure as follows:

Calculated or Indicated airspeed is indicated airspeed corrected for instrument errors, position error (due to incorrect pressure at the static port) and installation errors.

Calibrated airspeed values less than the speed of sound at standard sea level (661.4788 knots) are calculated as follows:

minus position and installation error correction.

Stay tuned for upcoming Blogs

## Airspeeds- Selecting a Replacement Indicator

Airspeeds – Selecting a Replacement Indicator

Typically, you can replace any airspeed indicator with another manufacturer’s indicator of the same type and range. However, there are some considerations, which should be taken into account.

Aerosonic and Kollsman indicators often have a longer case than other manufacturers, this could cause an installation problem in some aircraft.

On typical indicators the Pitot input will be in the center of the back of the indicator, with the Static input to the right, when viewed from the rear. Occasionally you will find some older indicators where this was reversed. This may cause installation difficulties on some aircraft.

TGH Aviation offers custom range markings for all blank-dial instruments (most commonly, factory new units) and re-screening services on used instruments in need of refreshed or adjusted range markings. Call or email one of our sales rep’s today for assistance and quotes on all of your Airspeed or other instrument requirements.

What would cause my airspeed indicator to be stuck at zero all the time?

A common cause of this is a problem with the Pitot source. If the line from the Pitot tube has disconnected, then pressure will not be applied to the instrument.

If the Pitot tube is clogged, then pressure will not be applied to the instrument.
It is common for Leaf Bugs to build nests in Pitot tubes and effectively clog them.

What would cause my indicator to be stuck at a high speed all the time?

A common cause of this is over pressure being applied to the Pitot tube. Aircraft are often washed using a high-pressure water hose. A careless mechanic may spray the hose across the Pitot tube applying high pressure to the air speed indicator. This will usually destroy the diaphragm. Units damaged in this way normally cannot be repaired and must be replaced.

Can you re-screen and/or range mark my dial?

Yes, we can. The repair department will advise you of the fee for this process. Screening dials is not done in house, this function is done at an outside vendor. The repair shop will advise the lead time. In order to ensure that the proper range marks are applied we require a copy of the airspeed specifications that are in the Pilot’s Operating Handbook.

The temperature bulb on my true airspeed indicator is broken can you fix it

No, we cannot. The bulb on a true air speed indicator is filled with gas. All of the gas escaped when the bulb broke. The bulb must be replaced. However, these bulbs are extremely expensive and extremely difficult to find.
Is the indicator you are selling me, Tso’d ?

United Instruments, Edo-Aire, Kollsman, Garwin, Aerosonic – Yes

UMA – No

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 name plate then you must assume that it is not approved to the TSO.

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.

What is the tolerance (accuracy) on an airspeed indicator?

Typically +/-2 (Knots or Miles per hour) up to 200 after which it is typically +/-3.

Why does it cost so much to overhaul my airspeed indicator?

Overhauling an airspeed indicator is not as simple as you would think. A typical overhaul includes all of the following steps:

Total dis-assembly and inspection.
Determine and correct the cause of failure.
Ultrasonic cleaning of all jewels, pivots, and gears.
Reassemble internal components.
Silk-screen dial and/or range mark
Calibrate the unit to manufacturers and/or FAA specifications
Assemble unit into case.
Seal Case.
Verify calibration of final assembly.
Accomplish cosmetic touch-up.
Complete all FAA mandated paperwork and certifications.

The typical time required to overhaul an airspeed indicator can range from 1 hour for a simple unit to over 3 hours for complex units.

Calibration of an airspeed indicator is a complex process. Airspeed is a non-linear function, which must be displayed on a linear dial. There are at minimum seven calibration points that are all interactive with each other.

## Airspeeds – Get to Know the Operation and Manufactures

Airspeeds – Get to Know the Operation and Manufactures

How it works

Pitot pressure is forced into the diaphragm causing it to expand like a balloon. Static pressure is contained within the indicator case and surrounding the diaphragm. As the static pressure changes it, will either cause the diaphragm to compress, as the aircraft loses altitude or allow it to expand as the aircraft gains altitude. This expansion and contraction of the diaphragm is mechanically linked to the pointer causing it to move around the dial thereby displaying the speed of the aircraft as a function of the difference between the Pitot and static pressures.

Range Marks

Range marks are a reminder to the pilot of the aircrafts basic operating envelope as it pertains to airspeed. Typical range marks found on an air speed dial are:

White Arc – VFE This is the maximum speed at which the aircraft can operate safely with the flaps extended.

Green Arc –  This is the normal operating range

Yellow (Orange) Arc – Caution

Red Radial – VNE Never exceed speed

Blue Radial – This is the minimum operating speed using one engine on a two engine aircraft.

Note: Maneuvering speed is not marked on the dial, it is normally on a placard, which is located on the instrument panel.

Manufactures

The following companies all have manufactured air speed indicators and are the most common that you will see:
United Instruments

Kollsman
Aerosonic
Aeromarine
McCleod
UMA
Garwin
Edo-Aire

## Airspeeds- Basic Principle of Operation

Airspeeds- Basic Principle of Operation

Main Components

The major components of an air speed indicator are:

1. Case
2. Diaphragm
3. Dial
4. Pointer
6. Hair Spring
7. Jewels and Pivots

Case
A standard air speed indicator for general aviation comes in a 31/8  inch 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.

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.

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

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 31/8” 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

1. Indicated Air Speed Indicator
2. True Air Speed Indicator (Manually Operated)
3. True Air Speed Indicator (Automatically Operated)
4. Maximum Allowable Air Speed Indicator
5. Mach Air Speed Indicator
6. F1 Air Speed Indicator
7. Helicopter Air Speed Indicator
8. 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.

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?

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

## How to Read a 3 Pointer Altimeter

##### 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