King Air 100 A/B
Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor
Flight Crew Last 90 Days 50; Flight Crew Total 3200; Flight Crew Type 1000
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Human Factors; Weather
On arrival to APA I was on radar vectors and descended down to 9000 feet.  I was advised that there would be light mixed ice below 9000 feet.  After 10 minutes or so of being vectored in the ice (temp was -10 and picking up moderate mixed) I was vectored for the final approach course between FIRPI and CASSE on the ILS to RWY 35.  Upon receiving clearance for the approach to maintain 8000 feet till established; I proceeded to intercept the localizer.  I noticed an intermittent localizer but proceeded to intercept.  I flew through the course by not much when the needle came alive.  Once turned back to intercept; ATC advised that it appears that you have flown thru the localizer.  I concurred and indicated a live needle.  I was still getting errors in indication and both the localizer and glide slope came in and out.  Still outside of CASSE; I reported intermittent signal to ATC with the mental plan of going missed once inside of the FAF.  I was told to report to tower.  Approaching the FAF I noted a glide slope indication suggesting I was high.  I began to descend.  Moments later; I looked down on the approach plate; confirmed my location (outside of CASSE) and concluded I was receiving an error indication and proceeded to climb and maintain 8000 feet as cleared.  At that moment; ATC transmitted; 'I'm receiving a low altitude warning.'  I responded; 'correcting'.  I was then reminded to maintain 8000 feet till established.  Upon reaching CASSE; I proceeded inbound with positive indication.  Interesting to note that upon reporting the intermittent indications; the aircraft ahead on the same approach confirmed the error sensing and gave a positive pilot report.  I proceeded inbound with little trouble thereafter with the RWY in sight at 6950 MSL.  Upon landing; I noted excessive ice accumulation on the aircraft surfaces and antennas in the amount of >1.5 inches and upwards to 2 inches in places.  It is my belief that the errors were the result of interference with the antenna signals for both aircraft.  Not certain...could have also been a weak signal.
A King Air B100 pilot reported getting low on the ILS to APA airport while flying in icing conditions and experienced erratic localizer and glideslope signals; possibly related to ice accumulation.