Pitot and Static System

The Pressure System

The pitot-static pressure system provides the source of air pressure for the

• ALTIMETER (ALT)
• VERTICAL SPEED INDICATOR (VSI)
• AIRSPEED INDICATOR (ASI)
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The two parts of the system are:

1. the pitot tube and pressure line
2. the static pressure system and lines

The pitot tube is normally mounted on the leading edge of a wing. The pitot tube on an aircraft used only for flight under Visual Flight Rules (VFR) may not be heated to prevent icing. Aircraft to be used under Instrument Flight Rules (IFR) are heated electrically, to prevent icing when operating in visible moisture and cold temperatures. A switch in the cockpit controls Pitot heat.

The static pressure port is normally found on the side of the fuselage. On later model aircraft, an alternate static source is provided inside the cockpit. The pilot can select the internal static source if the outside source becomes clogged with ice.. When the pilot selects the alternative source, the instruments relying on the static pressure may operate slightly differently.

THE ALTIMETER

The Altimeter (ALT) allows the pilot to determine the height above Mean Sea Level (MSL). Correct altitude indication is very important for several reasons.

a) The pilot must be sure the aircraft is being flown high enough to clear terrain and other obstacles.

b) The pilot must maintain altitude according to certain air traffic rules and instructions to minimize the possibility of mid-air collision.

c) The pilot can often select more favorable winds at certain altitudes.

d) True Airspeed calculation requires that the altitude be known.

The Altimeter measures the pressure of the outside air. A small bellows inside the altimeter which contains a constant pressure inside expands when the aircraft climbs, and contracts when the aircraft decends. This bellows is connected to a gear arrangement which causes the hands to turn as the bellows expands or contracts. The altimeter is essentially a barometer which is measuring the outside air pressure, but the indications on the dial indicate hundreds and thousands of feet.

Most altimeters have either 2 or 3 pointers. If 2 pointers, the longer one indicates hundreds of feet, and the shorter pointer indicates thousands of feet. A third very short pointer which indicates ten's of thousands altitude exist on some altimeters. The indication in the diagram shown below is 1,430 feet.

Altimeter Setting

The altimeter is an aneroid barometer. It is correct only when in a known atmosphere. An International Standard Atmosphere has been defined as a barometric Sea Level pressure of 29.92 inches of Mercury (Hg), and a temperature of 15 degrees Celsius.

Effects of Atmospheric Pressure Changes

Whenever the altimeter is in a non-standard temperature and pressure the altimeter reads incorrectly, and an adjustment means must be provided to compensate for the non-standard conditions. Atmospheric pressure change has the greatest effect on the instrument.

On modern altimeters, an adjusting knob and scale is provided to allow adjustment for non-standard pressure. In the diagram of the altimeter face above, a window on the right of the instrument shows a graduated barometric scale, called the altimeter setting. The pilot can adjust the altimeter setting with the knob on the lower left. The setting shown is 30.00.

If the altimeter is not periodically readjusted to the local barometric pressure, the plane will be too high if the local sea level pressure pressure is higher than 29.92 and will be too low if the local sea level pressure is less than 29.92 in. Hg.

As one flies cross country, the altimeter should be adjusted every 100 miles or so. If flying from a low to a higher pressure area, the aircraft will be higher than indicated if appropriate altimeter adjustment is not made periodically. For example, if the altimeter indicates 5000 feet, it will actually be above 5000 feet when in the higher pressure area.

Likewise, when flying from a high pressure area to a lower pressure area, the aircraft will be falsely low if no adjustment is made. If the altimeter is indicating 5000 feet, the aircraft will be below 5000 feet when in the lower pressure area..

The altimeter setting can be obtained in flight from any Air Traffic Control facility or from any FAA Flight Service Station. If taking off from an airport where no contact can be made with such a facility, set the altimeter to the altitude of the airport prior to takeoff.

When flying at or above 18,000 feet (Flight Level 180) the altimeter must be set to 29.92. These altitudes are primarially used by fast jet aircraft. Since there is no possibility of ground collision, all aircraft operate with the same altimeter setting.

Effects of Temperature

Temperature affects the indicated altitude. The effect is not as drastic as pressure changes. Altimeters in small aircraft have no simple means to compensate for non-standard temperature. The effect is similar to high and low pressure changes. When going from low temperature to higher temperature, the aircraft will be higher than the indicated altitude. When going from high temperature to low, the aircraft will be lower than indicated on the altimeter. The pilot should keep this in mind if terrain clearance is a factor in the flight.

Vertical Speed Indicator (VSI)

The mechanism of the VSI is similar to the altimeter except the bellows contains a small calibrated hole that allows the pressure inside the bellows to slowly adjust to the same pressure as in the case. Therefore the pressure inside the bellows is similar to what it was a few seconds ago.

If the change in pressure is slow, the up or down reading will be small. If the up or down altitude change is large over a short time, the rate of climb or decent will be large. If the pressure both inside and outside the bellows stays the same, the VSI will indicate zero.

The single pointer indicates level flight (indicating 0), climb in feet per minute ( pointer deflected upward), and decent in feet per minute (pointer deflected downward).

In small piston engine powered aircraft, the rate of climb will usually be less that 1000 feet per minute. Usual rate of decent enroute or approach to landing will be in the 500 feet per minute range. When flying straight and level, the instrument should indicate zero. Also when sitting stable on the ground the instrument should indicate zero. Most instruments are equipped with a small adjusting screw to calibrate the zero position when the aircraft is at rest on the ground.

Airspeed Indicator (ASI)

The Airspeed Indicator (ASI) measures the speed of the aircraft through the air. This should not be confused with groundspeed. Winds can affect how fast the aircraft tracks over the ground. Groundspeed is seldom the same as airspeed.

Principle of Operation

"Impact" air hiting the opening of the pitot tube which is pointing in the direction of travel creates a pressure in the pitot tube line. This pressure is connected by a small tube to the inside of a bellows in the ASI instrument. The outside atmospheric static pressure enters the case from the static line. The mechanism inside the ASI therefore measures the difference between the pitot pressure and the static pressure. Since the impact pressure of the pitot tube is proportional to the speed through the air, the speed through the air is indicated by the instrument.

Indicated Airspeed (IAS)

The color coding of the airspeed indicator has meaning. The color arcs are as follows.

White Arc - Stall Speeds and Flap operating Range

• Lower end of arc is the Power Off Stall speed with flaps and landing gear in the landing position.
• Upper end of arc is the maximum flaps extend speed.

Green Arc - Normal operating airspeed range

• Lower end of arc is the power off stall speed clean (flaps and gear up)
• Upper end of arc is maximum structural cruise speed. (Max normal operating speed).

Yellow Arc - Caution range. Avoid this area unless in smooth air.

Red Line - Never exceed speed.

• This is the maximum speed at which the aircraft can operate safely.
• It should never be intentionally exceeded.

Calibrated Airspeed (CAS)

Although aircraft designers attempt to keep airspeed errors to a minimum, it is not possible to achieve complete accuracy throughout the complete range of the instrument. Two types of errors can be introduced.

a. Installation error caused by the static ports sensing erroneous pressure. This is due to the unpredictability of the effects of the slipstream around the aircraft at various speeds and attitudes.

b. The pitot tube does not always present the same frontal appearance to the atmosphere at varying attitudes.

The pilot should consult the Pilot Operating Handbook (POH) for the table applicable to the aircraft being flown.

True Airspeed (TAS)

As altitude increases , air density decreases. The impact pressure at the port of the pitot tube is less at higher altitudes. The airplane is actually traveling through the air faster than indicated on the ASI. Consequently as altitude increase, Indicated Airspeed decreases.

A mathematical correction factor must be applied to Indicated Airspeed (or Calibrated Airspeed) to arrive at a correct True Airspeed (TAS). This calculation can be made with he E6B Flight computer, or an approximate correction can be made by adding 2 percent per 1,000 feet of altitude to the IAS.

EXAMPLE: Given IAS is 140kt and ALT is 6,000 feet. Find TAS.

2% x 6 = 12% (.12)
140 x 0.12 = 16.8
140 + 16.8 = 156.8 kt. (TAS)

Some airspeed indicators have built-in adjustment scales that allows the pilot to adjust the instrument for temperature and pressure. Both the IAS and TAS can be read from such an airspeed indicator.

V Speeds

The Pilot Operating Handbook normally lists various airspeeds for differing situations and conditions. The definition of the usual V speeds is shown below as an abbreviation for Velocity.