Alaska Airlines First with Telex Interphone Upgrade
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Alaska Airlines First with Telex Interphone Upgrade

September 29th, 2008


It’s something we’ve all seen when stepping on board a 737: the flight attendant stretches the cord on her handset around the corner of the cabin interior, making space to let passengers board the aircraft while she makes her announcements…


The B737-300/400 standard Telex HS-500 forward cabin interphone is mounted on a main thoroughfare wall. The design of the aircraft itself means the handset cord needs to be pulled as far as possible from the handset hanger to allow people to pass while the unit is in use. This extreme stretching leads to intermittent handset failures, posing obvious safety issues as well as incurring cost through servicing and replacement.

Though Boeing has a fix for this, which involves completely replacing that whole section of the aircraft, Roger Caldwell, Project Consultant & Avionics Engineer for Alaska Airlines, sought a less invasive and less expensive solution, and contacted Telex to discuss fitting new panels with longer cords to resolve the issue with no excess labor and cost. 40 of Alaska Airline’s B737-400s now have the Telex HS-500 OEM upgrade, an application-specific solution developed with Telex.


  • A handset loop assembly was designed using a machined piece part fabricated with more robust material to eliminate breakage.

  • Handset hanger hook pivot points were reworked by examination and installation of new thru-hole fastener hardware, thus correcting hanger vertical movement and microswitch actuation. The handset hanger hook as provided in the original installation is quite robust and has experienced few failures, thus redesign was not deemed necessary.

  • The handset coiled cord was replaced with a slightly lengthened/highly flexible cord to allow a maximum extended length of about eight feet based upon redesigned cord material as provided by the Telex Communications Aviation Handsets Group.


“The primary program objective of the new installation was ease of use by flight attendant personnel,“ says Roger Caldwell. “Inoperative handsets, frustration over too-short or inflexible coiled cable, and extended length of reach are all addressed and resolved per the upgrade. The cord now stretches 8-10 feet, so the flight attendant can stand on the other side of the airplane or well into the first class area while making announcements. The cord/handset shows no signs of strain and easily handles the stretch distance-yet curls up for neat stowage.

“The secondary program objective was directed toward simplicity of installation,“ Caldwell adds, “and the use of easily obtainable parts and materials, robustness, and ease of post-installation maintenance. Additionally, the Engineering Order indicates existing IPC detail to assist line maintenance personnel performing on-wing upgrades. Notably, since installation of the upgrade kit, all failures as related to the forward lavatory handset have been eliminated and the subject has been removed from statistical reliability reporting at Alaska Airlines.“

“We worked closely with R.H. Caldwell & Associates on this project, modifying our product to their specifications and saving the airline a considerable amount of time and money as a result,“ reports Steve Parker, Product Manager, Telex Aviation Products. “Alaska Air is the first success story with this OEM upgrade, and we now have an industry-wide solution available for any airline flying one of the 4000-plus 737’s in use.“