AN ACR FO WATCHED ANOTHER ACR ACFT LOSE CTL; REGAIN CTL; THEN FLY ON.

Narrative

I OBSERVED ANOTHER AIRLINE'S TRANSPORT CATEGORY TWIN-ENG JET ENTER INTO AN UNUSUAL ATTITUDE; LOSE CTL; AND RECOVER. WHEN FIRST OBSERVED AT APPROX XX47Z; THE SUBJECT ACFT APPEARED TO HAVE BEEN ON APCH TO RWY 26; APPROX 2 MI FROM TOUCHDOWN. OUR ACFT WAS LOCATED ABOUT 100 YARDS FROM THE BEGINNING OF RWY 25 ON THE PARALLEL TXWY FACING E. THIS POS PLACED US IN FULL VIEW OF THE OCCURRENCE. THE SUBJECT ACFT CAUGHT MY ATTN AND I COMMENTED TO THE CAPT; 'LOOK AT THAT GUY; HE'S CLBING LIKE A ROCKET!' THE ACFT APPEARED TO BE IN A GAR ATTITUDE AND CLBING AGGRESSIVELY. FROM THAT POINT; THE ACFT'S PITCH ATTITUDE CONTINUED TO INCREASE TO SLIGHTLY BEYOND VERT. FORWARD AIRSPD AND VERT CLB REACHED '0.' THE ACFT THEN DID A TAIL SKID FOR ABOUT 2 SECONDS IN A NEAR VERT NOSE-UP ATTITUDE. IT THEN APPEARED TO FALL OFF TOWARD ITS L WING IN A SLIGHTLY INVERTED ATTITUDE. THE NOSE CONTINUED TO FALL WITH THE ACFT IN A SPIRAL; POSSIBLY A SPIN; TO A STRAIGHT DOWN NOSE-LOW ATTITUDE. AFTER A 'LONG' SECOND OR TWO; THE ACFT PULLED OUT AND RECOVERED TO STRAIGHT AND LEVEL FLT APPROX 90 DEGS L OF ITS ORIGINAL HDG. THE ACFT THEN RESUMED CTLED LEVEL FLT.