An apparent design problem with the airborne weather radar radomes in Hawker business jets could prevent the radars from accurately painting storm cells ahead, making them appear less hazardous than they really are, say pilots.

There are about 150 Raytheon Hawker 700s, 800s and 800XPs worldwide fitted with Rockwell Collins TWR-850 radar systems, including those with the Pro Line 21 avionics suite, according to Owen O’Mahony, who manages a flight department that has two Hawker 800s with the TWR-850. “These have an antenna power of 24 or 32 watts,” he said. “The remainder of all Hawkers are fitted with Honeywell high-power radar, comparatively speaking, but even they require a minimum of a Class C radome. The problem is that the radome fitted to the Hawker is Class D.”

In an April letter to Charlie Riddle, Hawker program manager for the FAA’s Wichita aircraft certification office, FAA’s Hawker Program Manager, O’Mahony asked him to review the use of Rockwell Collins TWR-850 radar equipment with the existing Hawker radome. “This request arises as the result of a well documented aircraft incident and is highlighted by a recent revision to the radar OEM’s installation manual,” he wrote.

O’Mahony said the radome was designed in the 1970s, when the output of the weather radar was measured in kilowatts rather than watts. “In the 1980s, the British CAA approved an STC for the installation of the TWR-850 radar into the British Aerospace-built Hawker,” he said. “The output of the TWR-850-003 radar is just 24 watts.”

According to O’Mahony, in July last year, there was an incident in which a Raytheon Hawker 800XP was damaged when a highly experienced crew flew into a cumulonimbus cloud that did not show on the aircraft’s radar screen. He also cited another incident that occurred in VMC in which a cumulonimbus cloud directly ahead of the aircraft painted a green return on the radar screen. The green return changed to red when the aircraft altered its heading away from the cloud.

“Two field engineers, one from Raytheon and one from Rockwell Collins, subsequently flew on that particular aircraft and agreed with the crew and maintenance technicians that there were problems with the radar returns in certain sectors of the screen,” O’Mahony said. “They said they would build a ‘break out’ box to analyze the returns, but it was subsequently deemed unnecessary by Raytheon. But the company that manufactures the radome has confirmed that the design specification for the Hawker radome calls for an average transmissivity of 82 percent and a minimum of 76 percent–this would make it a Class D radome in accordance with RTCA/DO-213.”

O’Mahony said the Rockwell Collins installation manual for the TWR-850 radar requires a Class A radome. “When I talked to people at Rockwell Collins, they seemed surprised to find that the Hawker had a Class D radome.” As an immediate result, the company issued “Temporary Amendment No. 10” last November, which states: “Collins recommends the use of a Class A radome. Use of a lower classification of radome…degrades the performance of a radar system and is likely to result in extraneous and inaccurate weather depictions.”

“What Rockwell Collins’ statement is clearly saying is that if you do not have a Class A radome fitted for the TWR-850 radar, then don’t take any notice of the indications on the radar screen,” O’Mahony said. “The fact that both the FAA and the UK CAA have approved STCs for the installation of the Rockwell Collins TWR-850 radar raises several questions.

“Did the STC applicants make an accurate impact assessment of the Hawker radome specifically with respect to the TWR-850 low-power digital radar?” he asked. “Did the regulatory authorities ask for an impact assessment of the Hawker radome on the performance of the TWR-850 radar? Were the regulatory authorities given accurate information about any such impact assessment? And why was Rockwell Collins’s requirement for a Class A radome ignored or overridden?”

A spokeswoman for Rockwell-Collins said, “We publish a recommendation for radar requirements and the OEM makes the decision on what to use. The OEM works with the regulatory authorities to evaluate if it’s appropriate.”

A spokesman for Raytheon Aircraft said, “Due to possible litigation issues, we can’t comment at this time.” And a telephone call to the FAA’s Riddle resulted in a return call from a deputy for public affairs at the FAA Great Lakes Region office. “We’re currently investigating [O’Mahony’s] letter” was her official comment.

Through his Web site dedicated to thunderstorm knowledge and avoidance (www.av-wx-rdr.com), Archie Trammell of Trinidad, Texas- based Radar Training Systems, received a question. It asked, “A friend in the UK tells me the radar in his Hawker is very weak. Ours, same radar type in a Citation, works great. What’s the deal?” Trammell replied, “Bad radome.”

In his response, which Trammell shared with AIN, he said, “This subject simply will not go away. Ignorance–nay, stupidity–in the industry in relation to radomes is beyond belief. It begins with airframe manufacturers and continues through flight department managers–both airline and corporate, and follows throughout the FAA and JAA.

“In the case of the Hawker it’s an old, old story. I first ran into it 20 or more years ago. A client operating a Hawker told me his radar was no good. It was a 10,000-watt radar. He’d had it bench checked by a couple of shops and both found all of the electronics to be well within specs. I suggested he try a loaner radome from Norton. The difference was night and day.

“I later learned that all Hawker radomes are most likely bad. From back when Hawkers were manufactured in the UK, the factory specification for the radome has been Class D. My Collins installation manual calls for a Class A radome for the TWR-850. Even the 10,000-watt Honeywell Primus 440/660/ 880 radar series needs a Class C radome to work as intended. But, quoting from the 440/660/880 installation manual, ‘For optimum long-range performance, a class A radome is recommended.’

“But a radome’s class is not even half the story,” he continued. “Class refers to the one-way transmissivity of the radar’s transmitted energy. That’s the industry standard measurement for radome efficiency. But a more important question is: in what direction does the transmitted energy go?

“It’s supposed to go out in a cone-shaped beam. But if light fixtures or other hardware are installed on or inside the radome (a la Hawker) or if the nose shape of the aircraft is a sexy point (a la Hawker, Embraers and many Cessna piston twins), energy might be reflected into side lobes, often referred to as ‘side-lobe losses,’” he explained. “In that event the radar will be weak compared to its design specifications.”

How can you tell if you have side-lobe losses? “If you see echoes on your radar where there are no thunderstorms, you have side-lobe losses. The fact is, most corporate aircraft have bad radomes, as do many airliners.

“Bottom line on the subject,” Trammell concluded, “the radome is a critical part of the radar antenna. It must be tuned to the radar’s operating frequency. That tuning is a matter of very carefully controlling the thickness of the radome structure. The tolerance is only 0.005 of an inch. That’s the equivalent of only two coats of paint.”