At London City Airport (LCY) there exists a "CAUTION YOUR BLAST" sign. This isn't one I've seen before. What does it mean?
I'm fairly sure that sign is beside taxiway C*, and as you can see from the aerodrome chart it is right next to the main apron. Aircraft holding in this position have the main apron directly behind them.
The sign is instructing pilots to use the minimum thrust necessary to move off from that position, as there could be personnel, vehicles etc on the apron. As this is a bit of a mouthful "Caution your blast" is used.
This is further backed up by the aerodrome textual data which states:
Pilots are requested to use minimum power when manoeuvring on and off parking stands and when entering the runway. The use of minimum power is particularly emphasised when holding at the the entry points to the runway and when entering the runway.
The other hold points around the apron seem to have a similar problem Taxiway B has the same sign. Perhaps the others do too.
* This was based on personal memory of seeing this exact sign, and of being aware at which hold point we were at. I went and checked using google earth/street view. There is a good view from the terminal however the sign itself is obscured by an aircraft. However you can clearly see the building opposite from the original picture, and so the position for Taxiway C lines up.
Jet aircraft are legendary for generating enough exhaust force behind them to blow over people, baggage carts, smaller aircraft, or any other loose object. An extended NASA safety reporting system discussion of the hazard is at https://asrs.arc.nasa.gov/publications/directline/dl6_blast.htm. Boeing further describes blast details at http://www.boeing.com/commercial/aeromagazine/aero_06/textonly/s02txt.html, noting that the hazard extends hundreds of feet behind the aircraft's engines. When a jet has been stopped and needs to start moving, the crew needs to set power to reach "breakaway thrust", which is substantial for a large aircraft. Once moving, power can be reduced - but there will still be a blast hazard close behind a taxiing aircraft. In the environment described in the LCY question, all the runway entry points are likely to have stopped aircraft that need to go to breakaway thrust to start moving onto the runway, and that could easily cause control problems for aircraft moving on the apron behind them.