AN ACR DC10 FREIGHTER FLC DSNDED BELOW THEIR ASSIGNED ALT WHEN THE CAPT CHANGED AUTOFLT MODES AT THE WRONG TIME REMOVING ITS ALT CAPTURE CAPABILITY.

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CAPT FLYING THE LEG USING AUTOPLT; WITH INS FOR NAV. CTR GAVE US DSCNT FROM FL350 TO FL280. CAPT BEGAN BRIEFING 'APCH IN USE' AT APPROX FL300. DSNDING THROUGH FL290 I MADE COMPANY CALL OF '29000 FT FOR 28000 FT.' CAPT ACKNOWLEDGED. THEN WE CONTINUED HIS 'BRIEF' WITH BOTH OF US LOOKING AT THE APCH CHARTS. AT FL276 CAPT SAW THE ALT AND 'ARRESTED' THE DSCNT AND CLBED UP TO OUR ASSIGNED ALT OF FL280. SIMULTANEOUSLY CTR CALLED AND SAID CONTINUE DSCNT TO FL260. I ACKNOWLEDGED. CAPT SAW THAT WE HAD PROGRAMMED THE AUTOPLT TO 'AUTO CAPTURE' THE SELECTED ALT 'FL280;' BUT HE HAD CHANGED MODES TO 'VERT SPD' THEREFORE ELIMINATING HIS AUTO CAPTURE FUNCTION. LOOKING AT MY ARR APCH 'PLATE' I DIDN'T NOTICE THE LOSS OF 'ALT' SELECTED ON MY LIGHT PANEL FLT MODE ANNUNCIATOR. ANOTHER CASE OF NO ONE BACKING UP THE AUTOPLT. I SAW THAT ONCE I MADE MY '1000 FT' REMAINING CALL; I DIDN'T PAY ANY ATTN. SUPPLEMENTAL INFO FROM ACN 401610: THE FO QUICKLY REALIZED WHAT WAS HAPPENING AND ALERTED THE CAPT WHEN THE ACFT WAS AT FL276 (400 FT LOW). THE CAPT INSTANTLY REACTED TO STOP THE RATE OF DSCNT AND BROUGHT THE ACFT UP TO FL280. I DID NOT SEE THE ACFT DSND MORE THAN 450 FT BELOW ASSIGNED ALT.