Near midair collision between a saab SF340 on an ILS approach and a helicopter crossing underneath the localizer causing the SF34 TCASII alert to warn of traffic. The first officer flying the SF34 stopped descent and leveled off until the TCASII showed clear of traffic. Reporter believes that there could have been a collision if their aircraft did not have TCASII to warn them.

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Attributes
ACN	408235
Time
Date	199807
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : hgr
State Reference	MD
Altitude	msl bound lower : 1500 msl bound upper : 2000
Environment
Flight Conditions	Marginal
Light	Daylight
Aircraft 1
Controlling Facilities	tower : hgr tracon : dtw
Operator	common carrier : air carrier
Make Model Name	SF 340B
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	descent : approach
Flight Plan	IFR
Aircraft 2
Make Model Name	Helicopter
Operating Under FAR Part	other : unknown
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 150 flight time total : 3900 flight time type : 700
ASRS Report	408235
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	conflict : nmac other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : took evasive action
Consequence	Other
Miss Distance	horizontal : 0 vertical : 500
Supplementary
Primary Problem	Flight Crew Human Performance

Narrative:

Observed aircraft on TCASII while descending on an ILS approach. An obvious conflict was about to occur. Began to level the descent just as the TCASII called 'traffic, traffic.' a few seconds later, TCASII said 'monitor, vertical speed' indicating to stop descending, this traffic was below us. I believe the visibility to be below the reported 5 NM. If it was 5 NM, we most likely could have gotten a visual approach. The visibility was more like 2 or 3 mi. We could not see the field while being vectored on a right downwind. VFR aircraft in the vicinity were asking for 'practice ILS approachs' which probably means they could not see the airport. If our TCASII had been deferred (broken) that day (today), I estimate a 50%/50% chance of a midair collision with a nearly full aircraft. This tower needs some radar. ZDC has trouble seeing us through the mountains.

Original NASA ASRS Text

Title: NMAC BTWN A SAAB SF340 ON AN ILS APCH AND A HELI XING UNDERNEATH THE LOC CAUSING THE SF34 TCASII ALERT TO WARN OF TFC. THE FO FLYING THE SF34 STOPPED DSCNT AND LEVELED OFF UNTIL THE TCASII SHOWED CLR OF TFC. RPTR BELIEVES THAT THERE COULD HAVE BEEN A COLLISION IF THEIR ACFT DID NOT HAVE TCASII TO WARN THEM.

Narrative: OBSERVED ACFT ON TCASII WHILE DSNDING ON AN ILS APCH. AN OBVIOUS CONFLICT WAS ABOUT TO OCCUR. BEGAN TO LEVEL THE DSCNT JUST AS THE TCASII CALLED 'TFC, TFC.' A FEW SECONDS LATER, TCASII SAID 'MONITOR, VERT SPD' INDICATING TO STOP DSNDING, THIS TFC WAS BELOW US. I BELIEVE THE VISIBILITY TO BE BELOW THE RPTED 5 NM. IF IT WAS 5 NM, WE MOST LIKELY COULD HAVE GOTTEN A VISUAL APCH. THE VISIBILITY WAS MORE LIKE 2 OR 3 MI. WE COULD NOT SEE THE FIELD WHILE BEING VECTORED ON A R DOWNWIND. VFR ACFT IN THE VICINITY WERE ASKING FOR 'PRACTICE ILS APCHS' WHICH PROBABLY MEANS THEY COULD NOT SEE THE ARPT. IF OUR TCASII HAD BEEN DEFERRED (BROKEN) THAT DAY (TODAY), I ESTIMATE A 50%/50% CHANCE OF A MIDAIR COLLISION WITH A NEARLY FULL ACFT. THIS TWR NEEDS SOME RADAR. ZDC HAS TROUBLE SEEING US THROUGH THE MOUNTAINS.

Data retrieved from NASA's ASRS site as of July 2007 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.