FLC OF AN MLG RETURNED TO LAND OVERWT WHEN OBSERVING AFTER TKOF THAT THE IRS NAV SWITCHES WERE OFF AND THE HEADING INDICATORS WERE NOT ALIGNED WITH THE MAGNETIC COURSE INDICATOR. SUBSEQUENTLY; AFTER TAXI OUT FOR ANOTHER TKOF; AFTER OVERWT LNDG INSPECTION AND TURNING ON IRS NAV SWITCHES; THEY OBSERVED A SLAT FLAP INDICATION PROB AND RETURNED TO THE GATE. THEY FINALLY GOT AIRBORNE WITH EVERYTHING WORKING!

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CAPT FLYING THE ACFT VFR CONDITIONS; ON TKOF WE NOTICED HEADING FLAGS ON RMI AND HSI. AN OTHERWISE NORMAL TKOF AND CLB WAS MADE. WE SUBSEQUENTLY REALIZED THAT THE IRS- NAV SWITCHES WERE IN THE OFF POS AND WITHOUT AN ALIGNMENT WE WERE UNABLE TO NAV IN A NORMAL MANNER. THE CAPT THEN NOTIFIED ATC THAT WE HAD A NAV EQUIP PROB AND REQUESTED TO RETURN TO ARPT OF DEP. ATC THEN VECTORED US FOR A VISUAL APCH AND LNDG. DUE TO THE QUICK RETURN AN OVERWT LNDG WAS MADE AND NOTED IN THE ACFT LOGBOOK. WE TAXIED BACK TO THE GATE WITHOUT FURTHER PROBS. AN OVERWT LNDG INSPECTION WAS MADE BY MAINT AND THE ACFT WAS SIGNED OFF OK FOR FLT AND RE-RELEASED. ON TAXI OUT FOR THE SECOND TKOF WE NOTICED AN UNRELATED FLAP SLAT INDICATION PROB AND HAD TO RETURN TO THE GATE FOR CORRECTIVE MAINT. ONCE THE PROB WAS REPAIRED WE TAXIED OUT; TOOK OFF AND COMPLETED THE FLT NORMALLY WITHOUT ANY FURTHER PROBS. THIS INADVERTENT PROB PROBABLY COULD HAVE BEEN AVOIDED HAD WE USED GREATER CHKLIST DISCIPLINE AND VIGILANCE; DOUBLE CHKING EVERY ITEM.