A variometer is a very sensitive vertical speed indicator. France developed the Flaman Speed Indicator and Recorder. The speed indicator was fitted 6 September 1960. An increase in climb rate will not show up immediately on the vertical speed indicator. Some aircraft also have a taxi speed indicator for use on the ground. A Smith-Stone type speed indicator was installed 30 June 1960. It may be used for a speed indicator or in a feedback speed control system. Engine speed indicator takes speed indication also from this sensor. A Smith-Stone speed indicator was added on 15 October 1960. On large jet aircraft the IAS is by far the most important speed indicator. The vertical speed indicator, or VSI, is more of "a good help" than absolutely essential.