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Keeping Cool

By By Robert W. Moorman | April 1, 2016



Photo courtesy of Heli-One

Civil and military helicopter cockpits can get crowded with the addition of more avionics systems, and that crowding in the limited real estate behind an instrument panel combined with limited airflow can cause overheating.

Contributing to the issue is the growing use of software-centric systems in rotorcraft.

The problem of avionics overheating is not widespread. But instances of it have prompted operators, lessors and helicopter manufacturers to take corrective action to ensure that multi-million avionics systems are properly cooled.

Such issues also have led avionics makers to consider designing systems to run cooler.

Take the case of the Boeing CH-47F. The latest variant of the heavy-lift transport includes a digital cockpit, advanced communications and new avionics. The U.S. Army reported instances of avionics and other systems overheating onboard. That led to corrective measures to ensure that the Chinook’s electronics operate at the current qualified operating temperatures. These included increasing airflow and reducing radiant heat.

The CH-47F includes a digital cockpit, advanced communications and new avionics. Photo courtesy of Randy Tisor

Engineers at the Army’s Redstone Arsenal have reduced temperatures around the CH-47F’s control display unit by introducing ventilation into the center console via louver vents on the canted console, according to officials at the Army post southwest of Huntsville, Alabama. Blanket cutouts have been added to promote better air circulation and eliminate localized equipment hot spots.

The engineers also developed adjustable, multi-functional air diverter vents, which deflect air behind the -47F’s multi-function displays to improve airflow. The vents are installed on the outlet of the cockpit foot-warmer ducts.

Finally, an elbow duct was added to provide airflow above the Chinook’s equipment in the avionics compartment/electronics bay.

CH-47Fs in the field are being modified while new deliveries will include the cooling solutions.

Another military aircraft, the Army’s Boeing AH-64, experienced reliability problems with its environmental cooling system on the -64D Apache and -64E Apache Guardian. Fairchild Controls was hired to replace those systems.

It is unclear whether the previous cooling systems’ reliability issues caused or contributed to avionics overheating on the AH-64Ds and Es.

Boeing converted the avionics in AH-64Ds and Es to a vapor-cycle cooling system. Photo courtesy of the U.S. Army / Chief Warrant Officer 3 Mark Leung

The AH-64Ds and Es were converted to a vapor-cycle cooling system, according to Boeing. This increased the capability to cool more avionics and give flight crews better control of the environment in the Apache’s crew station. The new system provides pressurized cooling air to each Apache extended forward avionics bay, said Boeing.

The OEM said the efforts of Fairchild Controls (which is now part of the Triumph Group as Triumph Thermal Systems) fixed the cooling problems; the helicopter maker added that it is not aware of any current Apache cooling issues.

The U.S. Navy also is concerned with properly cooling avionics and other systems onboard its rotorcraft. Its Naval Air Systems Command is conducting a trade study of Marine Corps AH-1Z attack and UH-1Y utility helicopters “to characterize the equipment thermal environment and to propose design solutions for the integration of new avionics,” according to the command.

NAVAIR said that “thermal design is increasingly a concern” and the “growth in equipment loads parallels the growth of software-centric systems.”

The H-1 aircraft are challenged by their design. NAVAIR officials explained that the attack Vipers’ and utility Venoms’ relatively small electronics bays rely on convective cooling methods. But, officials said, those bays are now being filled by modern digital avionics with high heat loads.

Vector Aerospace is somewhat unique in the aviation business. It is an MRO and lessor. The company offers on short-to-medium-term lease Airbus Helicopters AS332L Super Pumas, which are backed by its OEM-approved Bypass and Bypass Plus by-the-hour support programs.

Avionics and other in-cabin systems in one batch of Super Pumas operating in hot-high environments in the Middle East have been reported to overheat partly due to the prevalence of fine talcum powder-like sand in the region.

Vector considered installing small-capacity portable air conditioning units on seat rails in the cabin. Cold air would have been ducted into the cabin and cockpit to cool the crew and avionics. A filtration system would have helped keep the avionics clean and reduce avionics overheating.

But the operator’s lease on the aircraft expired before that fix was implemented.

A few years earlier, however, a similar situation occurred on government-owned Sikorsky Aircraft S-61s and Bell Helicopter UH-1Hs operated for the U.S. State Dept. in the Middle East. Some of these helicopters were outfitted with avionics cooling systems installed by Vector.

‘A Huge Challenge’

Vector said it’s close to obtaining a supplemental type certificate for the S-61 cooling fix. An offshoot could be applicable to other rotorcraft, both civil and military, said the company.

“It is a huge challenge to keep the avionics functioning in these conditions,” said Elvis Moniz, Vector’s VP of business development for airframe and avionics solutions. “We have experienced problems with both older civil and military aircraft operating in these environments.”

Moniz said overheating could occur on any helicopter, even those equipped with digital avionics that don’t come with cooling fans. In some cases, the instrument panel can be enlarged to allow for better airflow, but that modification potentially limits the visibility of the pilot on approach.

Vector has been asked by rotorcraft operators in Asia to develop avionics cooling solutions, said Moniz, but he declined to name those operators.

Adding avionics to an already crowded cockpit or electronics bay compounds the challenge. The new boxes “bring additional heat load in the cockpit, cabin or avionics bay,” said Ed Demas, director of business development at Triumph Thermal Systems-Maryland (the name of the former Fairchild Controls). “It is easy to address the heat generated by each component. The picture becomes complicated as more components are added.”

Demas added: “Those components can be an integrated system where the integrator typically specifies a thermal operating environment and supplemental cooling can be provided. If components are not added as an integrated system, the user or the user’s modification provider takes on the task of heat management.”

Demas said solutions for carrying heat away can vary, for example by attaching heat generators to the structure or passing air over them. Managing heat loads becomes more complex when using a transport loop to carry the heat expelled overboard, he said.

(Cooling solutions need not be complicated. For instance, the use of windscreen covers on U.S. Marine Corps Bell Boeing MV-22s deployed in hot areas has helped reduce temperatures in the aircraft that can be elevated by direct sunlight.)

Although overheating of avionics on civil and military helicopters is not at a crisis stage, manufacturers and cooling systems providers are designing new cooler avionics systems or ensuring that their current production systems are properly cooled.

Universal Avionics provides “cold-wall cooling” instead of cooling fans in its products. Photo courtesy of Universal Avionics
 

Universal Avionics, which provides avionics for civil and military rotorcraft, does not offer cooling fans with its systems. Instead, it provides a “cold-wall cooling” system, said Robert Clare, director of sales. It also provides a proprietary power supply design that operates at a reduced temperature.

To achieve cold-wall cooling, space is designed between the front (display assembly) of Universal’s electronic flight instruments and the rear (card cage assembly), where internal flow is directed from both ends, said Clare. Bottom air inlets and fan assemblies help force air up through this space. Air is pushed out through a forced-flow, channeled air outlet and directed over external air flow and channeled across cooling fins.

“This all results in air being blown across heat sinks instead of components,” said Clare.

Cold-wall is similar to cold-plate technology in that heat is carried away by coolant. However, its configuration is different, said Triumph’s Demas. In cold-wall technology, circuit card assemblies are cooled by coolant passing through the walls to which they are mounted.

Other avionics manufacturers are looking at cold-plate or cold-wall technology. “Honeywell would be interested in any rotorcraft applications that may consider a liquid cooling loop and to resolve overheating electronics issues,” said George Sciacca, senior manager of technical sales for Honeywell Defense and Space’s Mechanical Systems. “That could be any methodology that would transport the cooling output of our environmental control unit to the source of heat, be it via air or liquid cooling.”

Honeywell considered installing cold-plate technology as part of its avionics offerings for military and civil rotorcraft, but no application has been found yet, said Sciacca.

Asked if overheating of avionics onboard rotorcraft is a concern, Sciacca said: “We haven’t been asked to add cooling units to our avionics. But we’ve heard of instances of avionics overheating, especially in upgrades when an operator wants to go with a more modern suite of electronics. We’ve heard of situations where avionics bays become more cramped and hotter.”

Avionics manufacturers are not involved directly in the ducting or actual cooling of the avionics systems typically. Nor do they design the tubes or manifolds around the electronics; that job goes to a vendor, an OEM-approved maintenance provider. Avionics makers are responsible for meeting the overall heat load mandated by the helicopter manufacturer, said Sciacca.

Running Cooler

Eric Christianson, Honeywell’s director of marketing and product management, said whatever solutions are created to cool avionics and other systems should be within reason: “Our primary goal is not to do something that is very expensive.”

On Rockwell Collins military avionics, temperature limitations were set initially for 55 deg C (131 deg F) and 71 deg C (159.8 deg F) in the short term. When overheating problems in hot conditions surfaced, the company re-qualified avionics at 71 deg C for continuous operations.

“Heat has not been a recurring problem with helicopter avionics except in the extreme temperatures during summer time, when the heat loading exceeds design temperatures of the avionics boxes,” said Daniel Toy, Rockwell Collins’ manager of rotor-wing marketing.

Rockwell Collins will, in the near future, unveil new efficient processors and graphics engines that would run on less power, resulting in multi-function displays that run cooler, said Toy. No further information was given.

External fans are not used to cool Rockwell Collins avionics, typically. The equipment is designed with integral fans that provide all required cooling, said Toy.

So-called “slim-profile” avionics help keep the temperature within limits, said Toy. They also allow for better circulation of air and provide room for a small cooling fan, if deemed necessary, he added.

Technofan, a division of Labinal Power Systems, part of the Safran Group, makes fans and valves to cool avionics and other systems for fixed- and rotary-wing aircraft. The company provides cooling fans for the Bell 429 and 407 and cooling devices for the Sikorsky H-92, the military variant of the S-92, as well as the new Boeing AH-6i. The fan sits on the floor between the two pilots and cools avionics via ducting. The company also provides cooling fans for the majority of Airbus and AgustaWestland helicopters.

For the 429, Everett, Washington-based Technofan Inc. provides a small pancake-shaped fan to cool avionics and other systems onboard. With a growing need to cool additional avionics onboard rotorcraft, one might surmise that business was brisk.

“I wouldn’t say business was picking up. It is a little bit challenging on the civil side,” said Rick Freeman, president of Technofan. “There is such pressure to keep costs down on the civil side that [OEMs] are not willing to invest in new or advanced technologies to properly cool avionics that are generating more heat.”

The industry is well aware of the overheating concerns of operators.

“We have noticed that it has gotten a lot hotter in the avionics bays,” said Rob Longfellow, an electrical engineer with Robinson Helicopter. But, as part of its rotorcraft certification, Robinson must demonstrate that all avionics meet the aircraft temperature envelope, he added.

Each avionics device is evaluated to ensure that the avionics limitation is not exceeded when installed in a helicopter with an outside air temperature of 50 deg C and taking into account the additional greenhouse heating effects inside the cabin, said Longfellow.

Finmeccanica’s AgustaWestland helicopters are designed to operate at a maximum temperature of 55 deg C (131 deg F) in continuous operation, in the absence of external forced airflow (such as natural convection cooling), according to the company. It said this engineering approach has prevented overheating issues.

The issue of cooling avionics systems onboard rotorcraft is likely to become more challenging. The FAA stated it wants to see more safety-related equipment onboard helicopters, particularly those serving the EMS market. And operators of small, single-engine helicopters are asking for autopilots to be installed. Robinson said it’s considering an autopilot for its single-engine R22.


Certification Update: HTAWS

Industry officials are working with the FAA to clear the way for operators to keep using installed terrain warning systems after April 2017.

The Air Medical Operators Assn. and others are developing a summary document for comparing the capabilities of installed enhanced ground-proximity warning systems (EGPWS) with the 2014 Helicopter Air Ambulance rule’s requirements for use of helicopter terrain awareness and warning systems (HTAWS).

Helicopter operators in 2006 began installing EGPWS, which were adapted from fixed-wing systems. The 2014 rule specifically requires installation—by April 24, 2017—of devices labeled as HTAWS that comply with Technical Standard Order C194 and related RTCA minimum operational performance standards. TSO-C194 was published in 2008.

The “checklist” being developed by the industry for FAA acceptance is aimed at satisfying agency officials that EGPWS installations meet the 2014 rule’s performance requirements for things such as hazard depiction and annunciation even though they pre-date TSO-C194.

The FAA’s acceptance would avoid requiring operators to replace their EGPWS with new HTAWS units.

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