Putting Data to the Test
Air Data Testing Systems are a crucial piece of aviation test equipment for use in testing and verifying aircraft flight instruments whilst the aircraft is firmly on the ground.
Aviation is a very precision and safety-conscious focussed industry that has become increasingly dependent upon accurate and real-time data for safe and efficient operations of flight.
Today’s modern aircraft rely on a plethora of air data components like the altimeter, airspeed indicator, vertical speed indicator (VSI), pitot tubes, static ports, and temperature probes, to generate navigation parameters such as pressure altitude, baro-corrected altitude, altitude rate of change, computed and true airspeeds (TAS), Mach number, total air temperature (TAT) and static air temperature (SAT).
The fidelity of these systems is tested and verified by Air Data Testing Systems (ADTS) or Pitot-Static Testers, an item of ground support equipment used to simulate airspeed, instantaneous vertical speed (IVSI) and altitude for aircraft line- and base-maintenance checks. Their versatility allows them to be used across different aviation sectors, making them suitable for a variety of airframes, including commercial airliners, business jets, general aviation aircraft and military aircraft and avionics systems.
As Chris Roberts, Product Leader – Test and Calibration at Druck, a Baker Hughes business describes, ADTS are available in two main configurations. The first is as a flightline product which has a built-in pump and is self-contained. This type of instrument can be directly connected to an aircraft. The second is as a workshop/laboratory product for manufacturing or testing avionic instruments not installed in an aircraft.
Check-ups
According to Maria Quezada, Marketing Manager at Laversab, the primary functions of the ADTS are to simulate flight conditions by controlling pressure in the aircraft’s pitot-static system, thereby allowing technicians to test these instruments in a controlled environment.
Paul Hart, Chief Technical Officer at DEA Specialised Airborne Operations expounds further. “An ADTS contains high-precision pressure and vacuum pumps within a portable ruggedised housing designed for flightline environments. The units connect to the pitot and static ports using special adaptors and colour coded tubing. The ADTS is typically controlled by a remote handheld unit by the maintenance engineer from the cockpit to cross-check that the airspeed, altitude and vertical speed indications on the electronic flight instrument system (EFIS (or “classic” round dial mechanical instruments)) correspond with the simulated pressures generated by the ADTS. The periodicity of this testing is stipulated by the aircraft manufacturer, who in turn are following guidance material from the regulations, such as FAA Advisory Circular AC43-60. Typically, this is a maximum of every two years or scheduled with routine maintenance inputs, such as a C-Check.”
ADTS also addresses common issues such as instrument calibration errors, leaks in the pitot-static system, and incorrect readings that could lead to unsafe flight operations.
“ADTS is also used when any defects are reported, either from the crew Tech Log or captured in the Central Maintenance Computer whenever a comparator mismatch has been detected between the Captain and First Officer EFIS displays, if discrepancies have been detected on airspeed, altitude or IVSI indications either side,” explains Hart. “A common issue is whenever a pitot probe is blocked from icing, bird strike or damaged during ground handling.”
In December 2021, an S7 Airlines A321neo suffered from reduced controllability after entering an area with severe icing after takeoff. It was subsequently determined that icing on three air data sensors caused a disturbed airflow to the pitot, static and angle-of-attack probes.
As a consequence the no. 1, 2 and 3 air data reference (ADR) used faulty airspeed and altitude data.
This would cause the system to underread and the combination of a visual check for the probe being misaligned, as well as a leak test using the ADTS would normally identify any fault rapidly,” Hart says.
Hart believes that ADTS are an essential item of Ground Support equipment, given that they perform an essential test function on a primary aircraft system during routine checks and after repairs. Quezada agrees they have become integral to a maintenance team’s avionics equipment portfolio. “Their role is crucial in ensuring that all air data instruments are functioning correctly before an aircraft is cleared for flight, making them indispensable for maintaining flight safety,” she says.
Crucially, there are some regions whose regulatory requirements mandate periodic testing of aircraft air data systems. ADTS help ensure compliance with these regulations.
In fashion
Since their introduction, ADTS have evolved as technology and requirements have progressed.
“The main developments over the years have been in the size of the products,” states Roberts. “In the early days they used to be built into trolleys and carts as they were so large. Nowadays they are about the size of carry-on luggage.”
Early ADTS during the 1940s-1970s used manual pumps to pressurise a volume within the test set. Under these conditions says Hart, the user would refer to a look-up table of pressures (in PSI – pounds per square inch) that corresponded to different airspeeds. Subsequently, these were replaced with electro-pneumatic pumps and since the 1980s these have been computer controlled to meter very precise pressures to test a sequence of airspeed and altitude conditions.
Modern systems are now more automated, allowing for quicker and more accurate testing procedures. Most recently the remote controls used by the engineer in the cockpit use Wi-Fi or Bluetooth connections to replace the coiled cables that have been used for some decades, making them more versatile and user-friendly.
“These improvements have enhanced ground maintenance by reducing the time required for testing and calibration, increasing accuracy, and minimising the risk of human error,” asserts Quezada.
Hart identifies essential features as being able to meet the airspeed, altitude and IVSI tolerances specified by the aircraft manufacturer.
Standout features of modern ADTS include Wi-Fi-enabled remote operation and Bluetooth connectivity, which according to Evolution Measurement, further enhances the efficiency of these devices. Technicians can connect wirelessly to the test sets, streamlining the testing process and eliminating the need for cables that can be cumbersome and prone to entanglement.
Other qualities embrace options for 2-, 3- or even 4-channel configurations. These systems also offer battery-powered operation, making them portable and convenient for use in various locations, from the hangar to the ramp, thanks to environmental ruggedisation – they are often exposed to bad weather on the flightline and need to tolerate “rough” handling, says Hart.
Roberts highlights the properties of Baker Hughes’ Druck ADTS units, which in addition to their measurement accuracy and stability, control performance and operating environment capabilities, can be controlled from a remote hand-terminal so during the testing programme the technician does not need to leave the cockpit. These in conjunction with the pre-defined test sequence capabilities means that test programmes can be completed simply and efficiently.
While ADTS have evolved significantly with advancements in technology, manufacturers must be aware of and contend with obsolescence and updates. For example, as Quezada explains, Laversab offers hardware and software upgrades for their ADTS. These upgrades ensure that the equipment remains compatible with the latest aviation standards and technologies. Additionally, Laversab provides support services, including calibration and loaner units, to ensure that customers can maintain their equipment’s performance over time.
Roberts says that Druck’s ADTS are subject to regular lifecycle reviews and refreshes to ensure they can be relied upon in the harshest environments. Any software updates can be carried out quickly and efficiently by the end-user.
RVSM acceptance
Manufacturers must also manage RVSM (Reduced Vertical Separation Minima) compliance.
As ATEQ, whose entire range of air data test sets is RVSM compliant, explains RVSM is a vertical separation standard between aircraft that was phased between 1997 and 2005 in air traffic management. It is the reduction of the standard vertical separation required between aircraft flying between FL290 (29,000 feet) and FL410 (41,000 feet) inclusive, from 2,000 feet to 1,000 feet (or between 8,850 and 12,500 metres, from 600 metres to 300 metres). To be RVSM compliant, the aircraft must verify that it is flying within these limits. Therefore, to achieve this accreditation, each aircraft must carry out a series of tests to confirm that all their pitot probes and flight indicators provide sufficiently accurate information.
RVSM compliance is thus essential for ADTS, remarks Quezada as it ensures that the aircraft can safely operate in airspace where reduced vertical separation standards are applied. “This compliance is critical for maintaining accurate altitude measurements, which is especially important in crowded airspaces where precise altitude data is necessary to prevent collisions,” she says.
Hart agrees, although he makes the distinction that while it is vital for aircraft that are operated above 29,000ft / FL290 and are required to maintain 1,000ft vertical separation, for aircraft such as regional turboprops with ceilings significantly lower than 29,000ft, general aviation and helicopters, this level of altimetry accuracy is not needed.
To obtain RVSM approval, operators must ensure that the aircraft meets minimum monitoring requirements established by their respective state authority (determined by the country where the aircraft is registered), pilots and crews must be trained in appropriate RVSM flight procedures, and the integrity and accuracy of the aircraft’s altitude-indicating systems must be closely monitored.
Monitoring flight checks must be completed every two years or 1,000 flight hours (whichever is greater) to maintain RVSM approval. When performing systems checks on aircraft equipped with RVSM-certified altimeters, it is imperative to use RVSM-compliant air data test sets and pitot-static test equipment.
The likes of ATEQ, DMA, Druck, Laversab, and Raptor Scientific amongst others offer ADTS that achieve this accreditation.
Calibration time
Maintaining the fidelity, integrity and accuracy of these systems requires regular testing and rigorous design and manufacturing processes, including the use of high-precision sensors and components. ADTS manufacturers will typically calibrate pressure systems against a US National Institute of Standards and Technology (NIST)-certified reference.
“Druck specialises in pressure control and measurement. We manufacture and design our own sensors so are in control of the whole process,” said Roberts. “By using our own unique design of sensor, TERPS, we can ensure the specification we state is adhered to with no degradation of performance over the specified interval. Unlike some of the sensing technology in the market, TERPS is not density sensitive for example. We have also spent many years developing our control technology and when coupled with our sensing capabilities customers can trust the accuracy and stability of our ADTS units.”
Baker Hughes’s Druck ADTS units are uniquely configurable and can be automated to complete pre-defined tests sequences for airspeed, altitude and angle of attack.
Laversab says it is systems can perform various test sequences, such as leak checks, accuracy checks for altimeters and airspeed indicators, and simulations of different flight conditions to test the performance of air data computers
For the ADTS to stay accurate, a calibration check should be carried out at chosen intervals. This is used to check the calibration, verifying the accuracy of the pressure sensors and ensuring that the system’s readings are within specified tolerances, without adjusting it. It may be used either to see if the ADTS requires a calibration or to verify performance following a main calibration. If the accuracy of the ADTS is not within the specification, it is recommended to carry out a main calibration, which adjusts the accuracy of the main transducers.
“As the equipment manufacturer we cannot tell a customer how often to calibrate their equipment,” says Roberts. “That is defined in their processes and procedures. We do however publish a specification for each unit, this can be as long as an 18 months specification due to the capabilities of our sensor technology, which can help reduce cost of ownership.
“Using PACE Tallis, our new transfer standard, calibration of the test sets is a simple and easy process and can potentially be completed just about anywhere. Customers may assess that they no longer need to perform calibration in an laboratory or workshop due to PACE Tallis not being as susceptible to environmental effects as traditional calibration instruments.”
The appropriate interval between recalibrations is generally recommended every 12 months, although as Hart says, intervals depend on specific aircraft and can range from monthly (when there is a specific airworthiness directive) to every two years. ATEQ Aviation, for example, recommends that its instruments be calibrated annually to maintain optimal accuracy.
Calibration of an ADTS can be performed at a manufacturer’s facilities or by authorised service centres, ensuring that the equipment maintains its precision and reliability. Laversab offers two-day and four0-day options for instance.
Another key benefit is the multitude of test sequences these devices offer. These sequences are carefully designed to assess various parameters, ensuring that every crucial aspect of air data is thoroughly tested. From altitude and airspeed measurements, leak testing and EPR testing, modern air data test sets can accommodate a wide range of testing requirements.
In addition to the primary air data system tests, ADTS can perform a range of other testing requirements, says Quezada, such as verifying the performance of the aircraft’s pitot-static system, testing the air data computer, and checking the functionality of altimeters and airspeed indicators under simulated flight conditions. These systems can also perform leak tests to ensure that the pitot-static system is airtight and free from defects that could impact flight safety.
ADTS units can also be used to carry out cabin air pressure switch tests and have other applications such as checking altitude alerters, low airspeed warnings, outflow valves and cabin depressurisation sensors that would trigger oxygen mask deployment in an emergency.
Selection advise
With such an array of units available, what makes an ADTS a suitable candidate?
For Quezada, when selecting an ADTS, it is important to consider factors such as the type of aircraft you will be testing, the required level of accuracy, and whether RVSM compliance is needed. Additionally, consider the system’s portability, ease of use, and available support services, such as calibration and software updates. “Opting for a versatile and upgradeable system like those offered by Laversab ensures that your ADTS will meet current and future needs.”
Laversab offers both online and in-person training for users and free technical support for all customers. The company also provides loaner units to reduce downtime during maintenance, while all its ADTS units come with a three-year warranty, ensuring reliability and customer satisfaction.
Hart says that the ADTS must meet the accuracy and parameter range requirements specified by the aircraft OEM. “Manufacturers must provide a quick turnaround time when the ADTS is returned, typically annually, for sensor calibration.” He also identifies portability and durability for flightline environments. Also on his shopping list are self-test on power-up, overpressure detection to prevent damage to aircraft sensors, and for airliners (Boeing, Airbus), that they can simulate ohmic resistance from total air temperature probe to simulate Mach number in combination with the pitot and static pressures.
For Roberts, the main areas to focus on is how efficient is the volume control. “Very often you will be trying to generate a vacuum or pressure up through a system of hoses and pipes which can have a sizeable volume associated with it, especially if the ADTS is being used through a line switching unit or on multiple pitot static systems simultaneously or independently. Baker Hughes Druck ADTS units are market leading when it comes to accurately controlling pressure into a large volume.
“Secondly but just as important is to understand what factors are included in the accuracy specification of the units this should include. A clear definition of what is meant by the accuracy specification, confirmation that all factors are included in the accuracy specification and a clear definition of what is meant by the precision specification.
“Lastly, how well shielded from environmental affects is the ADTS – such as fluid density, fluid humidity, temperature and EMC (Electromagnetic Compatibility).”
By Alex Preston
