CANADAIR REGIONAL JET ON CLB WITH DEFERRED INOP GENERATOR; FLC HAD APU RUNNING FOR GENERATOR REQUIRED BY MEL. RPTR CAPT INADVERTENTLY SHUT DOWN APU WHEN MEANING TO SHUT OFF CONTINUOUS IGNITION. JUST PRIOR TO THAT THE FMS FAILED AND DEFAULTED TO DEST SO IT WAS UNUSABLE. FLC EVENTUALLY WAS ABLE TO GET APU BACK ON THE LINE.

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BUSY MORNING; PICKED UP AIRPLANE THAT HAD MULTIPLE DEFERRED ITEMS. WX WAS OVCST WITH SNOW. IT TOOK QUITE A WHILE TO SORT THROUGH ALL THE DEFERRED ITEMS AND THE ASSOCIATED LIMITATIONS; PROCS; ETC. THEN SOME MORE INVOLVED THAN NORMAL TKOF CALCULATIONS DUE TO WX MADE FOR A BUSY MORNING. 1 DEFERRED ITEM WAS THE #2 IDG (THIS RENDERED THE #2 ENG DRIVEN GENERATOR INOP). THIS MEL REQUIRED US TO LEAVE THE APU RUNNING AND ITS GENERATOR ONLINE. AFTER TKOF ON RWY 3; WE WERE ISSUED A R TURN BY DEP AND CLRED DIRECTLY TO DNV VOR (OUT OF NORMAL VOR RANGE); BUT ON OUR RNAV FLT PLAN. NO SOONER WERE THOSE WORDS OUT OF THE CTLR'S MOUTH WHEN OUR FMS BLINKED ONCE AND TOTALLY ERASED OUR FLT PLAN. IT DEFAULTED TO OUR DEST AND BECAME UNUSABLE. WE ASKED FOR A VECTOR TO DNV AND I PROCEEDED WITH CLB CHKLIST. I THEN MOVED TO THE CLB CHKLIST BEFORE ATTEMPTING TO DEAL WITH THE FMS. SINCE WE HAD TOPPED THE CLOUD LAYER AND SNOW SHORTLY AFTER TKOF; I ELECTED TO TURN OFF THE CONTINUOUS IGNITION; WHICH I HAD ON FOR TKOF. WHAT HAPPENED NEXT WAS STRANGE. I REACHED UP TO PUSH THE SWITCH FOR THE IGNITION; BUT INSTEAD PUSHED THE APU START/STOP BUTTON; SHUTTING DOWN THE APU. THE SWITCHES ARE CLOSE TO EACH OTHER AND AS I WAS PUSHING IT; I KNEW I HAD THE WRONG ONE BUT IT WAS TOO LATE. I DON'T KNOW IF IT WAS HABIT THAT CAUSED IT BECAUSE THAT WAS THE POINT WHERE THE PNF NORMALLY SHUTS DOWN THE APU AND I THINK MY HAND JUST WENT THAT WAY AUTOMATICALLY. ANYWAY; THAT BASICALLY JUST CUT OFF SOME MINOR ITEMS AND SOME GALLEY PWR (IT HAD US DOWN TO 1 GENERATOR DUE TO THE DEFERRED IDG). AFTER THINKING HOW STUPID THAT WAS; I GOT READY TO RESTART THE APU -- NO BIG PROB SO FAR; BUT GUESS WHAT? THE APU WOULD NOT RESTART. WE HAD A MESSAGE THERE WAS A FAULT WITH THE AIR INTAKE DOOR. WE CALLED OUR MAINT CTLR WHO PROMPTLY TOLD US HE COULD NOT HELP US WITH THE APU UNTIL WE REACHED OUR DEST. SO HERE WE WERE IN BEAUTIFULLY CLR SKIES ON 1 GENERATOR HEADING FOR AN IFR APCH. NOT REALLY SERIOUS AS WE STILL HAD AN AIR DRIVEN EMER GENERATOR IN CASE OUR LUCK CONTINUED AND GENERATOR #1 FAILED; BUT A LESS THAN IDEAL SIT NONETHELESS. LUCKILY; ENRTE WE WERE ABLE TO COAX THE APU TO START AND CONTINUED TO DEST WITHOUT FURTHER EVENT. LOOKING BACK ON ALL THIS; THE WORKLOAD GOT PRETTY HIGH AS ALL THIS HAPPENED IN A SHORT SPAN OF TIME. WHILE I WAS PRETTY HAPPY WITH THE WAY WE AS A CREW DEALT WITH IT; IT STILL WAS EMBARRASSING FOR ME IN REGARD TO MY BUTTON FAUX PAS. BUT; IT ALSO BROUGHT UP ONE OF MY PET PEEVES; AND THAT IS FLYING AIRPLANES WITH MULTIPLE DEFERRED ITEMS. THEORETICALLY; UNLESS A SPECIFIC MEL ITEM HAS A LIMITATION; THERE REALLY ISN'T ANY LIMIT TO THE NUMBER OF ITEMS YOU CAN HAVE AND IT DOES GET VERY CONFUSING IF YOU HAVE A NUMBER OF ITEMS (ESPECIALLY IF THEY HAVE OPERATIONAL REQUIREMENTS THAT ARE NOT NORMAL AS IS OFTEN THE CASE). PRETTY SOON YOU ARE BASICALLY OPERATING A DIFFERENT AIRPLANE WITH DIFFERENT PROCS FROM THE ONES YOU TRAIN WITH. NOW SOME MIGHT MAKE THE ARGUMENT THAT A REAL PLT COULD DEAL WITH ALL THIS. I REALLY DON'T BUY THAT. BUT MORE IMPORTANTLY; AND WHAT I SAW THIS PARTICULAR MORNING; WAS THAT ALL BETS ARE OFF IF SOMETHING ELSE GOES WRONG. WITH NUMEROUS (OR EVEN ONE MAJOR) SYS(S) INOP; YOU'RE IN A LESS THAN IDEAL SIT ALREADY AND IF SOMETHING ELSE FAILS OR GOES WRONG; YOU COULD BE IN SERIOUS TROUBLE. I DON'T KNOW IF FAA SHOULD LIMIT THE NUMBER OF DEFERRED ITEMS THAT CAN BE CARRIED AT ONE TIME; BUT IT MIGHT BE WORTH INVESTIGATING. THAT IS WHY I SUBMIT THIS TO YOU FOR ENTRY INTO THE DATABASE. MAYBE SOME OTHER CREWS HAVE EXPRESSED CONCERN WITH THIS. NOTE: THE PREAMBLE TO OUR MEL DOES SAY THAT INTERRELATIONSHIPS BTWN ITEMS AND CREW WORKLOAD SHOULD BE CONSIDERED BUT GIVES NO SPECIFIC GUIDANCE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT CONFIRMED AS REGIONAL JET CANADAIR. RPTR CAPT WASN'T SURE WHY THEY HAD DIFFICULTY RESTARTING THE APU; BUT SAYS THERE HAVE BEEN INSTANCES OF THE APU CTL COMPUTER MALFUNCTIONING. APPARENTLY THERE WAS NO RELATIONSHIP BTWN THE DEFERRED INOP GENERATOR AND THE FMS ERASING THE FLT PLAN AND DEFAULTING TO THE DEST. RPTR SAYS THAT NEITHER OF THE FLC HAD INPUT ANYTHING INTO THE FMS PRIOR TO THE FAILURE. RPTR COMMENTS THAT HIS COMPANY HAS JUST RECENTLY BEEN FLYING THE RJ'S AND HE WAS SURPRISED HOW MUCH MORE INPUTS ARE REQUIRED AS COMPARED TO THE SAAB 340 WHICH HE HAD PREVIOUSLY FLOWN AND BOTH HAVE GLASS COCKPITS. RPTR CAPT MAKES AN INTERESTING STATEMENT: 'IN GLASS COCKPIT AIRPLANES; EVERYTHING HAPPENS VERY FAST AND EFFICIENT; INCLUDING ERRONEOUS INPUTS.'