STCs unnecessary for some TAWS installations

 - October 9, 2007, 6:54 AM

In a welcomed shift in policy, business aircraft operators may now forego the STC process when installing class-B terrain awareness and warning systems (TAWS). That was the word handed down by the FAA’s certification branch to FSDOs recently, published as a flight standards airworthiness bulletin (FSAW 02-03A) directed to avionics safety inspectors.

In the bulletin the FAA spelled out the procedure change, which allows hundreds of Part 91 and 135 operators affected by the TAWS rule to install the required equipment under the less stringent FAA Form 337 field-approval process–provided that an STC has been approved previously in a “similar installation.”

The previous approval does not have to involve the same model or type of aircraft, but should deal with “a comparison of interfaces” (sensors, displays and so forth) and “operational characteristics,” said the FAA. Inspectors in the field are being given latitude in deciding what installations are appropriate under Form 337, and observers expected that more and more field approvals would be granted as time goes on.

Avionics makers applauded the change, saying it eases the burden on operators of in-service turbine aircraft who otherwise would have had to obtain individual STCs.
“This is an important decision, and one that makes the installation process much easier for business aircraft operators,” said Gerry Block, president of Sandel Avionics of Vista, Calif., which makes both class-A and -B TAWS equipment.

Installations that include functionality going beyond the minimum class-B TAWS equipment requirements are also eligible for follow-on field approval, said the FAA. Additional functionality could include interfaces for a cockpit display, radio altimeter, landing gear, flaps, glide-slope and so forth. If this is the case, a more extensive installation evaluation and flight check must be performed, however.

If necessary, the bulletin stated, an FAA-conducted flight evaluation might be required to verify that “the design and installation perform their intended functions and that there are no adverse interactions between the TAWS and existing aircraft systems.” The local aircraft certification office or a designated engineering representative (DER) test pilot would conduct the flight evaluation.

Class-B TAWS will be required in all in-service Part 91 turbine-powered airplanes with six or more passenger seats and Part 135 turbine airplanes with six to nine seats starting on March 29, 2005. The process governing installation of class-A TAWS, required in all Part 121 airplanes and Part 135 airplanes with 10 or more seats, continues to specify individual STCs for each installation.

The FAA already requires that new airplanes be fitted with TAWS at the time of avionics installation through the TC process. Questions about the bulletin should be directed to the FAA at (202) 267-3809.