The dangers of using contaminated runways are a continuing concern in the aviation industry. A recent survey has reported on a total of 39 overrun and veer-off cases in the USA between 1962 and 1973 and in 26 of these cases the runway condition was cited as the major cause. The degradation of aircraft landing and take-off performance depends on the depth of water on the runway and thus an instrument is required that will give quick accurate remote estimates of runway water depth without hindering normal operational procedures. A suitable method, using a differential pressure transducer has undergone field tests and the instrument and preliminary results are discussed in this report.