Breakdown.

This is coolbert:

Consider with the context of previous blog entries here and here the susceptibility of the tank to mechanical breakdown. 

"World War I: The Tech of the Tank"

The article by Ellen Terrell and thanks to same.

MECHANICAL FAILURE FROM THE  INSTANCE OF THE FIRST EMPLOYMENT OF THE TANK IN WARFARE UNDERSTOOD AS A MAJOR PROBLEM. 

"On the morning of September 15th, 1916, the British Army attacked the German trenches just outside the French village of Flers. Forty-nine tanks were dispersed in small groups with the infantry divisions that made the assault. Even before the attack actually began, machines began to break down. More than a third of the tanks failed to reach their starting positions, and once the attacks started, still more broke, were delayed or became stuck, with the result that only nine achieved their goals"

THAT FINAL TALLY OF TANKS REACHING OBJECTIVE [GOAL] ONLY EIGHTEEN PERCENT [18 %] OF THOSE STARTING OUT. ALL FROM MECHANICAL BREAKDOWN? THIS IS NOT SPECIFIED. I MIGHT WELL IMAGINE THE "GOAL" NOT FAR FROM THE STARTING-POINT.

If indeed the "goal" to cross no-man's land in an unscathed matter while remaining operationally functional meaning to cross a stretch of land the width of from a quarter-mile [about 200 meters] to a half-mile [about 800 meters] the mechanical functioning the tank during the period leaving a tremendous lot to be desired.

coolbert.