B777-200 CREW HAD A FLAP OVERSPD DURING AN ENCOUNTER WITH WINDSHEAR AND GAR.

Narrative

AIRPLANE HELD AT BNN FOR APPROX 15 MINS; ATIS WAS MONITORED AND WINDS RPTED AS GUSTY FROM 200 DEGS; 23 KTS GUSTING TO 37 KT. DECISION WAS MADE TO LAND FLAPS 25 DEGS WITH AUTOTHROTTLES OFF AS PER FOM. AT APPROX 100 FT ON RWY 27R LHR; THE AIRSPD DROPPED FROM 157 KTS TO 142 KTS AND THE ACFT SANK TO ALL RED ON THE VASI. AN AURAL WINDSHEAR ADVISORY WAS RECEIVED SIMULTANEOUSLY WITH THE CREW INITIATING A GAR. THE ACFT DSNDED TO BELOW 50 FT BEFORE STARTING TO CLB. THE TOGA SWITCHES WERE ENGAGED AND THE CAPT WENT TO FIREWALL THRUST. AS SOON AS A POSITIVE RATE OF CLB WAS ESTABLISHED; THE GEAR WAS RAISED. AT APPROX 500 FT THE FLAPS WERE RAISED TO 20 DEGS. AS THE ACFT CLBED OUT OF THE SHEAR; THE AIRSPD JUMPED SIGNIFICANTLY. THE COPLT NOTED THAT THE AIRSPD WAS 20 KTS ABOVE FLAPS 20 DEG SPD. FLAPS WERE RAISED TO 15 DEGS AND IT WAS NOTED THAT THE AIRSPD WAS APPROX 30 KTS ABOVE 15 DEGS FLAPS SPD. PWR WAS REDUCED AND FLAPS RETRACTED ACFT THEN FLEW ANOTHER APCH AND LANDED SUCCESSFULLY ON RWY 27L. LOGBOOK WRITE WAS MADE FOR THE FLAP OVERSPD.