An air carrier mdt lost all transmitting capability.

Attributes
ACN	240939
Time
Date	199305
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : grb
State Reference	WI
Altitude	msl bound lower : 9000 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	Medium Transport, High Wing, 2 Turboprop Eng
Navigation In Use	Other Other
Flight Phase	cruise other
Route In Use	enroute airway : zmp
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial pilot : cfi pilot : atp
Experience	flight time last 90 days : 200 flight time total : 4400 flight time type : 200
ASRS Report	240939
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : declared emergency
Consequence	Other
Supplementary
Primary Problem	Aircraft

Narrative:

While in cruise flight, #1 radio began having transmitting problems. Within 5 mins, #1 was unusable. We informed ATC of our problem and asked for frequency change to contact company for further instructions on whether to proceed to grb. Company said affirmative. Upon recontacting ATC, #2 radio transmitter began to fail. We were unable to tell ATC before total failure. At this time we ran the communication failure checklist. We also began trying different mikes and the additional crew member audio panel to no avail. All things that were tried had no effect on our ability to transmit, though receive capability was never interrupted. We then decided it was time to squawk 7700 and identify. There was no response from center! After 1 min we squawked 7600 with no response. Grb WX was IFR and our departure airport of mqt was clear and visibility unlimited. After a few mins we reversed course and started flying the reverse back to mqt. After 5-10 mins, center contacted us to confirm our situation and intentions through squawk codes and identify. We then continued safely and landed at mqt without incident. The problem was a faulty rcau unit which processes all 3 audio panels transmitters. This seems to be unsafe and a design flaw which may warrant corrective action on the mdt aircraft. Because of this flaw there is no backup transmit capability when the rcau fails. A good lesson was learned that by following procedures set forth in the aim and good crew team work, we returned to our departure point without incident.

Original NASA ASRS Text

Title: AN ACR MDT LOST ALL XMITTING CAPABILITY.

Narrative: WHILE IN CRUISE FLT, #1 RADIO BEGAN HAVING XMITTING PROBLEMS. WITHIN 5 MINS, #1 WAS UNUSABLE. WE INFORMED ATC OF OUR PROB AND ASKED FOR FREQ CHANGE TO CONTACT COMPANY FOR FURTHER INSTRUCTIONS ON WHETHER TO PROCEED TO GRB. COMPANY SAID AFFIRMATIVE. UPON RECONTACTING ATC, #2 RADIO XMITTER BEGAN TO FAIL. WE WERE UNABLE TO TELL ATC BEFORE TOTAL FAILURE. AT THIS TIME WE RAN THE COM FAILURE CHKLIST. WE ALSO BEGAN TRYING DIFFERENT MIKES AND THE ADDITIONAL CREW MEMBER AUDIO PANEL TO NO AVAIL. ALL THINGS THAT WERE TRIED HAD NO EFFECT ON OUR ABILITY TO XMIT, THOUGH RECEIVE CAPABILITY WAS NEVER INTERRUPTED. WE THEN DECIDED IT WAS TIME TO SQUAWK 7700 AND IDENT. THERE WAS NO RESPONSE FROM CTR! AFTER 1 MIN WE SQUAWKED 7600 WITH NO RESPONSE. GRB WX WAS IFR AND OUR DEP ARPT OF MQT WAS CLR AND VISIBILITY UNLIMITED. AFTER A FEW MINS WE REVERSED COURSE AND STARTED FLYING THE REVERSE BACK TO MQT. AFTER 5-10 MINS, CTR CONTACTED US TO CONFIRM OUR SIT AND INTENTIONS THROUGH SQUAWK CODES AND IDENT. WE THEN CONTINUED SAFELY AND LANDED AT MQT WITHOUT INCIDENT. THE PROB WAS A FAULTY RCAU UNIT WHICH PROCESSES ALL 3 AUDIO PANELS XMITTERS. THIS SEEMS TO BE UNSAFE AND A DESIGN FLAW WHICH MAY WARRANT CORRECTIVE ACTION ON THE MDT ACFT. BECAUSE OF THIS FLAW THERE IS NO BACKUP XMIT CAPABILITY WHEN THE RCAU FAILS. A GOOD LESSON WAS LEARNED THAT BY FOLLOWING PROCS SET FORTH IN THE AIM AND GOOD CREW TEAM WORK, WE RETURNED TO OUR DEP POINT WITHOUT INCIDENT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.