CAP437 North Sea Meteorological data for Wind, Temperature, RH & Pressure
CAP437 is the Standard for Offshore Helicopter Landing Areas. CAP437 provides the criteria applied by the CAA in assessing the standards of offshore helicopter landing areas for worldwide use by helicopters registered in the United Kingdom. There are various parts to CAP437, however, this Application Note is going to focus on the meteorological data and specifically performance including wind, air temperature, humidity and pressure only.
Client Systems Use
Please note from the table that only WindSonic and WindObserver are able to provide the 4Hz output rate required for client systems to calculate the 3-second gust under WMO-No. 8 Seventh Edition 2008 ISBN 978-92-63-10008-5 also used for CAP437.
Relevant sections of WMO-No. 8 Seventh Edition 2008 ISBN 978-92-63-10008-5 are Chapter 5, sections 5.8.2 and 5.8.3
MaxiMet
MaxiMet has the option to provide customer selectable vector rolling average and 3 second gust in accordance with WMO-No. 8 Seventh Edition 2008 ISBN 978-92-63-10008-5, however, MaxiMet provides this using 1Hz. We accept that there may be up to 10% deviation in maximum gust values due to a peak falling between the 1 Hz outputs. We balance this reduction in sampling with the power savings of approx. 20mA @ 12VDC compared to running at 4 Hz.
MetPak
MetPak has the option to provide customer selectable vector rolling average and 3 second gust in accordance with WMO-No. 8 Seventh Edition 2008 ISBN 978-92-63-10008-5, however, MetPak provides this using 1Hz. We accept that there may be up to 10% deviation in maximum gust values due to a peak falling between the 1 Hz outputs. Please see KN1102 as this is not loaded as standard.
WindObserver
WindObserver has the option to provide customer selectable vector rolling average and 3 second gust in accordance with WMO-No. 8 Seventh Edition 2008 ISBN 978-92-63-10008-5, however, WindObserver provides this using 4Hz.
Specific Modes
Full details of the specific modes can be found in the manuals. The WindObserver mode is an example:
The direction and magnitude outputs are derived from the vector sum of U and V over the RWALONG averaging period (default 10 minutes in P1).
The gust output is derived from the vector sum of U and V over 3 readings (3 seconds in P1), and the max gust is the maximum of the gust value over the RWASHORT period (default 60 seconds in P1). The max gust value is reset to zero at the end of each RWASHORT period.
New issue of CAP 437, 8 December 2016
Please note that there are no changes to the specification requirements (shown in the tables in this Application Note) for wind, air temperature, and relative humidity and pressure sensors in this latest issue.
Air temperature, humidity and pressure
Previously we have focused on wind data and the table below focusses on other parameters that are measured by MaxiMet & MetPak.
Although MaxiMet and MetPak offer a compact single output solution; neither meets the criteria for pressure measurement under CAP437 as they only have one pressure sensor and requirements are for two or more that can be compared with each other.
The CAP437 requirement tables, and the product-by-product compliance grid for WindSonic, WindObserver, MaxiMet and MetPak, are in the original PDF linked below; they do not survive as page text.
Originally published as a Gill application note (AN0007).
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