Currently have a 3270 MAR router with a smart serial interface connected to a KIV 7 (military encryption device), then KLAS TA then INMARSAT SDU then goes from aircraft via Sat to land earth station to ground terminal where ISDN replaces INMARSAT equipment and 2811 with WIC 1T for ground router. Same setup otherwise. Serial protocal goes down when in flight even though the INMARSAT system maintains a connection and strong signal. Using ppp and pulse time 5. KIV 7 security device is just a pass thru and clocking comes from ISDN connection. Not sure if its vibration, network settings, cable losses, etc... when the A/C is in flight because it's works great when AC is stationary. Any suggestions on how to optimize this connection or better track down problems.