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:

 

pitot picture.jpegminus position and installation error correction.

 

Where


Vc
 is the calibrated airspeed,

 

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

 

P0is 29.92126 inches Hg; static air pressure at standard sea level,

 

a0is 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 forP0and   A0_smallare 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:

pitot picture.jpeg
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.

If the customer has requested a specific range and we do not have that range, you may offer a different range. However, you do not want to vary by a large amount. For example, an alternate range for 0-200 might be 0-180 or 0-240. Unacceptable alternates for 0-200 would be 0-150 or 0-300.

Airspeeds- Frequently Asked Questions

Airspeeds – Frequently Asked Questions

 

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
  5. Mechanical Linkage
  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.

 

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

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

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.

Getting to know your Kollsman Window and Barber Pole

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.

barber_pole