A king air B100 pilot reported getting low on the ILS to apa airport while flying in icing condition...

Narrative:

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 runway 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 runway 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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1243924
Time
Date	201503
Local Time Of Day	0601-1200
Place
Locale Reference	APA.Airport
State Reference	CO
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	King Air 100 A/B
Operating Under FAR Part	Part 135
Flight Phase	Final Approach
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Commercial Flight Crew Multiengine Flight Crew Instrument Flight Crew Flight Instructor
Experience	Flight Crew Last 90 Days 50 Flight Crew Total 3200 Flight Crew Type 1000
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Deviation - Track / Heading All Types Inflight Event / Encounter CFTT / CFIT Inflight Event / Encounter Weather / Turbulence

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.