Air carrier inadvertently taxied into the ILS critical area while an aircraft was on final approach.

Attributes
ACN	111134
Time
Date	198905
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : mht
State Reference	NH
Altitude	msl bound lower : 1600 msl bound upper : 1600
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : mht
Operator	general aviation : personal
Make Model Name	Small Aircraft, Low Wing, 1 Eng, Fixed Gear
Flight Phase	descent : approach descent other
Route In Use	arrival other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	ground other : taxi
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : atp pilot : cfi
Experience	flight time last 90 days : 150 flight time total : 3000 flight time type : 200
ASRS Report	111134
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

While on training flight for instrument rating, on final approach segment inside OM, approximately 1 NM and established on localizer and G/south, tower discovered, admonished and immediately told major air carrier aircraft to cross approach end of landing runway in order to clear the ILS critical area which the air carrier mistakenly taxied up to. Up to that point, our ILS indications were normal while the other aircraft held in the critical area. However, as aircraft taxied across runway, our needle indications became erratic to the point of mild concern over a possible missed approach. Ceiling was only 150-200' above minimums and although a training flight, real hard IMC conditions existed. Also in the rare event of the air carrier being disabled on the runway while attempting to cross it, with an aircraft on about a 3 mi hard IFR final, the remote possibility of a missed approach at best, or a collision at worst, especially with a communication failure on our part during the last minute of the approach, is admittedly very remote, but also very fatal.

Original NASA ASRS Text

Title: ACR INADVERTENTLY TAXIED INTO THE ILS CRITICAL AREA WHILE AN ACFT WAS ON FINAL APCH.

Narrative: WHILE ON TRNING FLT FOR INSTRUMENT RATING, ON FINAL APCH SEGMENT INSIDE OM, APPROX 1 NM AND ESTABLISHED ON LOC AND G/S, TWR DISCOVERED, ADMONISHED AND IMMEDIATELY TOLD MAJOR ACR ACFT TO CROSS APCH END OF LNDG RWY IN ORDER TO CLR THE ILS CRITICAL AREA WHICH THE ACR MISTAKENLY TAXIED UP TO. UP TO THAT POINT, OUR ILS INDICATIONS WERE NORMAL WHILE THE OTHER ACFT HELD IN THE CRITICAL AREA. HOWEVER, AS ACFT TAXIED ACROSS RWY, OUR NEEDLE INDICATIONS BECAME ERRATIC TO THE POINT OF MILD CONCERN OVER A POSSIBLE MISSED APCH. CEILING WAS ONLY 150-200' ABOVE MINIMUMS AND ALTHOUGH A TRNING FLT, REAL HARD IMC CONDITIONS EXISTED. ALSO IN THE RARE EVENT OF THE ACR BEING DISABLED ON THE RWY WHILE ATTEMPTING TO CROSS IT, WITH AN ACFT ON ABOUT A 3 MI HARD IFR FINAL, THE REMOTE POSSIBILITY OF A MISSED APCH AT BEST, OR A COLLISION AT WORST, ESPECIALLY WITH A COM FAILURE ON OUR PART DURING THE LAST MINUTE OF THE APCH, IS ADMITTEDLY VERY REMOTE, BUT ALSO VERY FATAL.

Data retrieved from NASA's ASRS site as of August 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.