Air Start Units/Carts Calibration & Repair

Ground support equipment (GSE) that provides pneumatic power to a parked aircraft and starts its engines is known as an Aircraft Ground Start Unit (ASU) or Cart. These mobile carts, which are often run on diesel-fueled IC engines, provide the correct air amount (in PPMs) to the aircraft’s receptacles at a constant pressure. The airflow is aided by an integrated dry screw compressor/gear-set for oil fee air delivery driven often by a Cummins or Detroit Series diesel engine, which is compatible to run with both jet fuel and diesel. ASUs are recognized for their endurance and can work in temperatures as low as negative Fahrenheit temperatures for up to eight hours when fueled entirely.


Why are ASU Carts necessary?

An aircraft engine’s starting mechanism is comparable to that of a car, incorporating a self-starter motor along with a starter button. The throttle may be used to regulate the speed once the engine is started. Similarly, ASUs serve the same purpose as pneumatic self-starters motors powered by high-pressure air supplied by the IC Engine in starting an Aircraft engine. The air start engine, unlike the automobile self-starter that is powered by battery power, the air start engine requires pressurized air to function. As a result, ASU is just as effective as a compressor. The electricity from an onboard engine may be tapped to start other engines after it has been started using an ASU.

Although not all aircraft require an ASU to be started, most have one as their standard procedure. APUs (Auxiliary Power Units) are also used by a few operators to provide the bleed air into the jet engine of an aircraft for the initial power-up.


ASU Units – Operational Principle:

The tow vehicles are driven up to an aircraft awaiting the anticipated departure time, where they are secured with a towing hitch. The discharge hose of the air start unit is connected to the aircraft’s high-pressure connection after receiving permission from the Pilot. The engine fan, compressor, and turbine blades begin rotating as a result of the power supplied by the air start unit once it is switched on. The engine’s flywheel turns at a steady rate until the pilot switches on the fuel injection system (of the onboard engine) to ignite the combustion chamber. Once enough heat is generated in the combustion chamber, the jet engine can run indefinitely, and the air start unit may be unplugged.

Most airliners use ASUs to start one jet engine (typically the one on the right and side of Pilot) and then another engine or engines (according to the aircraft configuration). Before pushback, most Pilots utilize an ASU to start just one of the jets before beginning taxiing, while the other engine is started during taxiing.

An air starter motor’s basic structure consists of an Engine, a Gear Mechanism, a Clutch, and a Turbine. The Clutch engages or releases the engine from the jet engine drive shaft (spool). When compressed air (at high pressure) is provided as input to the turbine, this air expands and leaves as low-pressure air out of the machine when it reaches its maximum capacity. The turbine converts pressurized air into mechanical energy and turns the motor shaft. It is also turned by virtue of its relationship with the jet engine’s shaft (or spool). When fuel is introduced into the combustion chamber, and a change in speed control from the air starter motor to the jet engine is required, the ASU’s motor clutch is released, allowing it to be disconnected from the jet engine’s spool. The connection between the air start unit and the hose is then cut off.


ASU Units – Types and Selection Criteria:

ASUs are classified into three types based on the low-pressure air supply source: a stored air cart, a gas turbine-based machine, and a diesel engine-powered screw compressor unit.

Classification based on delivery capacity: ASUs of various rates are available, ranging from 100PPM to 400PPM. The highest capacity ASUs currently on the market, which can start an engine in 48 seconds, are the 400 PPM (MAK) and 408 PPM (Rheinmetall), which are used to start the General Electric GE90-115B engines. The power produced by these units is nearly identical to that of a 747 APU START performance. Smaller commercial, passenger, or military aircraft engines may be started using less powerful ASUs.


Classification based on the power source: Depending on the source for low-pressure air delivery into the aircraft, ASUs are classified into three types – stored air cart, a gas turbine-based unit, and a diesel engine driven screw compressor unit.

  1. Stored Air Cart: A stored air cart is a reservoir of low-pressure air, often taken from an APU ( Auxiliary Power Unit) or a mobile ASU unit, and is commonly referred to as “bleed air.” The reservoir is placed on a cart and delivered to the aircraft via hoses.
  2. Gas turbine driven: These ASUs are driven by a GT engine and receive air from the atmosphere before being transformed into low-pressure, high-volume air to discharge into the plane. These ASUs are powered by electricity to turn the turbines, which function as an inverse of conventional (electric) driven air compressors. However, because of their low power to volume ratio, GT-driven ASUs are less popular among aircraft operators. These systems also need rigorous turbine blade maintenance and operational standards.
  3. Diesel engine driven: The name “Integrated Combustion Unit” (ICU) indicates that these units are linked to an IC Engine, which provides rotational energy to the turbines through the gearbox-shaft mechanism. Diesel engine-driven ASUs are most popular owing to their versatility in extreme weather conditions and portability. These do not require a battery or an electric power supply to operate, and they can even run on jet fuel unlinked GT-driven engines.


Classification based on the Mounting configuration: Based on the mounting configuration, ASUs are classified as Trailer-mounted, Skid-mounted, or Self-propelled.

  1. Trailer mounted units: Portable ASUs coupled to a ground support vehicle. Often, diesel-powered ASUs are mounted on trailers and are maneuvered to the aircraft.
  2. Skid Mounted: Four wheels are used to steer these ASUs, which are mounted and constructed on a metal framework. These devices may also be towed to a ground support cart or manually transported.
  3. Self-propelled units: The most typical ASUs for self-propelled vehicles are those that have been created (integrated) into the self-powered vehicle. Self-propelled units are the most popular mounting method for operators who want to start their engines.


ASU units – Selection criteria:

Factors such as capacity needs, energy efficiency, emissions, environmental effects, space usage, and operational efficiency should be considered while selecting an ASU unit.

  1. Capacity: The capacity of the ASU unit to power an aircraft stand is determined by the starting power requirement for an aircraft engine. The discharge ranges of ASUs range from 100 PPM to 400 (408)PPM. ASUs can be chosen based on the size and pneumatic power demand of the aircraft.
  2. Energy efficiency and Electricity requirements: GT ASUs require continuous and regulated electric supply to power the turbines, and hence, these systems are not portable, as opposed to the diesel engine driven ASUs, which are portable and are independent in operation. Also, due to its high body to power ratio, diesel-powered Operators often choose ASUs over the GT (Gas turbine) driven ASUs, which often require standard starting and stopping protocols.
  3. Emissions and Environmental constraints: Though Diesel-powered engines are a popular choice for ASUs, these engines are prone to emit SOx (Sulphuric oxides), NOx (Nitrous oxides), and other harmful gases into the atmosphere, which degrade the air quality. In cold, foggy environments and at night, using a diesel engine-driven, ASU is challenging due to PM (Particulate matter) emission, further hampering visibility. A stored air ASU or a GT Powered ASUs is effective in these situations.
  4. Operation Flexibility: Diesel-powered ASUs are preferred due to their flexibility in operation, over the stored air systems owing to less air pressure loss, and variable air discharge capacity. GT-powered ASUs have the least operation flexibility since they must be continuously fed electricity to operate.
  5. Maintenance ease: Because of the numerous starting and stopping functions, GT-powered units are difficult to maintain. Among all configurations, stored air systems are the simplest to maintain, followed by diesel-based systems.


Operational Guidelines & Preventative Maintenance of Water carts:

The FAA recognizes ASUs as ground support equipment, and proper maintenance and inspection are critical. The following are OEMs’ recommendations for using the Air starter carts:


  1. Filter Maintenance: Check the condition of the filters (fouling level, tears, corrosion) and the sealing gaskets (except for assembly with bare runners). Check for mechanical jamming of the filters, and clean all the components upstream of the filters and the duct networks, filter support frames, inside of the filtration box, and remove any impurities using a vacuum cleaner and/or a cleaning product, and ensure that there are no foreign objects inside the box. Filters can be replaced once the maximum pressure drop recommended by the manufacturer has been reached or in case of excessive leaks, damage, or corrosion. OEMs recommend maintenance at three-month intervals.
  2. Fans Maintenance: Check the motor/fan/mountings assembly for corrosion, check for noise and vibrations from the bearings, and check that the fan is clean. Fouled blades may lead to imbalance and breakdown of the fan. Flexible connections, anti-vibration mounts, and bolts can also be checked at every three-month intervals.
  3. Heating and Cooling Coils: Make sure the water inlet and outlet ducts are in good condition. Check the tightness of the connections and retighten if necessary. If the coil is not operating, we recommend bleeding it fully. Blow all the circuits with compressed air to bleed them fully. For glycol/water mix heat exchangers, check the level of frost protection using a refractometer. Cleanliness is a determining factor in the unit’s performance; therefore, carefully check the coil condition.
  4. Electric heaters: At every three-month interval, check for heater problems. If any, they can be removed for complete cleaning or sent for replacement.
  5. Steam humidifier: At annual intervals, clean and check the humidifier and replace its steam cylinder. Ensure that the humidity sensors are operating correctly and the fouling level of the eliminator cells is under limits. Check for corrosion or damage.
  6. Damper and mixing: Check the integrity and efficiency of the protective grilles and clean the dampers. Check the condition of the linkages and replace them if necessary, and ensure that the blades are straight at three-month intervals.
  7. Heat recovery units: This includes inspection and maintenance of plate recovery unit and rotary heat exchangers at three-month intervals.


Ground personnel must thoroughly study the operation and service manuals supplied by the equipment manufacturers. Every ASU machine employed should have a maintenance log, which must be completed at each servicing interval, detailing all work done on the system.

To the greatest extent feasible, aircraft operators should recognize that ASU carts are essential equipment and that failing to maintain proper hygiene and preventative maintenance standards will harm the engine or crucial aviation components. It is critical to supply clean and carefully controlled airpower when an ASU cart is connected to the aircraft.


e2b calibration offers industry-leading consultancy and certified PM services for your aircraft ASU carts. At e2b Calibration, we maintain and troubleshoot your carts so that you can concentrate on maintaining your aircraft. Our labs are ISO/IEC accredited and operated by a team of qualified experts providing training and consultancy services on AC cart maintenance and inspection. Our verifiable services are unmatched in the industry. Contact e2b calibration for all your equipment calibration needs.



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