What is an Air Data Test Set?
Air Data Test Sets, also known as Pitot-Static Testers, are highly specialized pressure generation and measurement instruments used to both
leak test an aircraft's pitot-static system as well as to test and verify an aircraft’s instruments by simulating the altitude and speed of aircraft while
still on the flight line.
These test sets are incredibly useful tools when calibrating instruments as part of an aircrafts’ inspection and maintenance regimen and to validate compliance
with FAA regulations such as providing documentation for Reduced Vertical Separation Minimum (RVSM) approvals.
A number of instruments used by both fixed and rotary wing aircraft of all sizes, rely on data gathered from pitot tubes as part of a pitot-static system. These
instruments display critical flight information including altitude, airspeed, rate of climb, manifold pressure, and engine pressure ratio (EPR). Additionally,
flight computers, sensors and pressure switches may rely on pitot tubes for reliable operation.
Pitot-Static System
A pitot-static system is a system of pressure-sensitive instruments used in aviation to determine critical flight data. Pitot-static systems consist primarily
of a pitot tube and static ports which extend through the skin of the aircraft, and instruments which operate via the principle
of air pressure gradient in which air pressure or pressure differences are used to calculate speed, altitude, and other flight parameters.
As critical flight information is gathered via an aircraft’s pitot-static system, it is absolutely essential that the system is functioning correctly and accurately.
Inaccurate readings can lead to dangerous situations. Several commercial airline disasters have been traced to a failure of the pitot-static system.
The primary elements of the pitot-static system are the pressure ports:
Pitot tube: Pitot tubes measure fluid flow velocity. In its basic configuration, pitot tubes are placed pointing directly into the fluid flow. The
fluid enters the opening at the front of the tube and stagnates as there is no outlet to allow the flow to continue. The stagnation pressure of the fluid, measured
by highly accurate pressure sensors within the pitot tube, is known as total pressure or pitot pressure (Pt) in aviation applications.
Static port: Static ports are small, flush mounted openings on an aircraft’s fuselage. The static port is connected to a highly accurate pressure
sensor that measures barometric pressure also known as static pressure (Ps) as it measures non-moving air pressure unlike pitot pressure which
measures moving air pressure. Since barometric pressures are lower at higher altitudes, static pressure will decrease as the aircraft climbs. Some aircraft have
more than one static port so that an average pressure can be taken, allowing for more accurate readings.
ADTS Functions
A number of critical flight instruments and systems can be tested with an Air Data Test Set. They include:
Airspeed indicator: Airspeed is determined by the difference between the ram air pressure from the pitot pressure and static pressure. This difference
is known as dynamic pressure. Higher airspeeds will create greater ram pressure and, therefore, higher dynamic pressure
Altimeter: Pressure altimeters make altitude measurements by measuring the difference between the pressure in an aneroid barometer inside the altimeter
and the static pressure. Since air pressure decreases with an increase of altitude at a relatively constant rate, altitude is simple to calculate as long as the altimeter
has been calibrated prior to flight.
Vertical speed indicator: The vertical speed indicator (VSI) measures an aircraft’s rate of climb or descent and is used to determine if the aircraft
is level in flight. The VSI is connected to the static port via a restricted diffuser and includes an internal diaphragm which is connected to the static port without
restriction. The vertical speeds is determined by the resulting differential pressure measurement between the instantaneous static pressure in the diaphragm and trapped
static pressure within the case. When the pressures are equalized in level flight, the needle reads zero.
Manifold pressure: Manifold pressure is the difference in air pressure between the engine intake manifold and atmospheric pressure in an internal
combustion engine. Measured as inches of mercury vacuum, it corresponds to the rate of airflow through an engine. Since an engine’s power is a function of how much air
and fuel flows into the cylinders, manifold pressure is one of the best methods of determining how much power is being produced by the engine. When an engine is shut
down, manifold pressure should be close to the atmospheric pressure.
Engine Pressure Ratio (EPR): The engine pressure ratio is a means of measuring the amount of thrust being produced by a jet engine. EPR is the ratio
of pressure measured by probes installed in the engine inlet and at the turbine exhaust. Since there is a limit to the pressure produced by an engine, EPR helps ensure
engine limitations are not exceeded.
ADTS Features
Air data test sets are sophisticated instruments that provide a wide range of functions and options. Though it’s not practical to list every feature of every ADTS unit,
there are some features that help distinguish one model from another:
Multiple channels: Basic ADTS units provide a single channel allow just a single measurement at a time. Multiple channel units, on the other hand,
allow simultaneous measurements and tests such as pitot-static air data validation, leak testing, fault-finding and avionics instrumentation testing making testing
protocols faster and more efficient.
Reduced Vertical Separation Minimum (RVSM) Approvals: RVSM or Reduced Vertical Separation Minima was implemented to reduce the vertical separation
between aircraft from a 2000 foot minimum to a 1000 foot minimum at altitudes from 29,000 to 41,000 feet. RVSM was designed to increase airspace capacity and access
to more fuel-efficient flight levels. ADTS units with RVSM approval meet high accuracy standards to ensure that flight instruments are properly calibrated and equipped
to handle flying in close proximity with other aircraft.
Pump capacity: Air data test sets test and verify the flight instruments of all types of aircraft, from fixed-wing to helicopters; light aircraft to
commercial jets. Since ADTS units are self-contained, they include both pressure measuring and generation capabilities. Pump capacity is an important consideration for
larger aircraft as a small pump is insufficient for providing enough pressure to simulate flight conditions.
Hand terminal: Some manufacturers have begun adding hand terminals making it easier to use the air data test set. These hand terminals may include
tablet-style touchscreens and wireless technology removing the need to run hoses and cables into the cockpit during testing.
Things to Consider When Purchasing an Air Data Test Set:
- Do you require multiple channels?
- What altitude range is required?
- What airspeed range is required?
- What specific measurements are needed?
- Is RVSM compliance needed?
- Is an EPR function needed?
- What accessories will be required?
If you have any questions regarding air data test sets, please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
