SA-227 AC Metro III
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 100; Flight Crew Total 1900; Flight Crew Type 75
Situational Awareness
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented; General Maintenance Action
Procedure; Weather; Aircraft
Aircraft was clear of ice through cruise and descent.  Approaching the airport and listening to the ATIS I knew icing conditions were present.  Given the aircraft is certified for flight into icing conditions and all de-icing and anti-icing equipment appeared to be working normally I proceeded with the approach.  Icing conditions were moderate clear to start; then moderate rime.  The de-ice boots were doing an excellent job of keep the wings clear.  Approaching the final approach fix for the ILS I noticed the pitch control was less responsive and that the trim was pitched further nose up than usual given the aft Center of Gravity from the heavy cargo load that morning.  The wings were clean; but I activated the boots again on the descent assuming that ice may still be present on the tail.  I landed 20 knots faster and with 1/4 flaps and that may have prevented a possible tail stall.  After shutdown and inspecting the tail section there was still roughly an inch and a half of ice along the entire leading edge of both sides of the horizontal stabilizer.  I squawked the condition in our maintenance log and had our maintenance department arrange contract maintenance at the airport.  No fault was found in the de-icing system.  The mechanic explained to me that if I let too much ice build; the boots won't be strong enough to break the surface clean; and signed off the maintenance log after an ops-check was normal.  I called one of the lead training staff for the aircraft as a resource and together we determined that the explanation didn't seam applicable given that I activated the boots with roughly a half inch of accumulation and several times after until landing.  He suggested that since maintenance could not find a fault; that the airplane should be airworthy; and to get a clearance to an altitude above the icing on the initial climb as a precaution.  I flew back without incident; where our in house maintenance performed a second inspection on the system.  The bleed air lines to the tail de-ice boots were dry rotted and leaking air.  I tempted fate twice by flying in icing conditions with inoperative de-ice equipment.  I should've waited out the icing conditions at the out station and flown back when there was no icing present.  That would've been a far better precaution to take until a second set of eyes looked at the de-icing system.  Given the aerodynamic effects of the ice present on the tail; if I remained in those conditions much longer; the end result may have been much worse.
SA227 pilot suspects tail icing during approach with the tail de ice activated and elects to land with 1/4 flaps and 20 knots faster. Post flight reveals an inch and a half of ice on the leading edge of the horizontal stabilizer. Contract Maintenance is called and can find no faults with the system and the return flight is without incident. A second inspection by company Maintenance finds dry rotted bleed air lines; preventing the system from operating properly.