I have noticed on a number of domestic flights recently some red streamers placed over the Pitot tubes on 738's. Presumably these are put there (and taken off again before push back) by the ground crew as a visual warning to avoid damage?
As explained by N860CR, the covers are to keep insects out of the probes. Normally they're put on overnight, but can be used any time there's a problem with insects. They certainly wouldn't work as warnings to ground crew....if anything, they'd home in on them. I'd have to admit that their proximity to aerobridges is pretty horrifying.
So, exactly are these probes that you're looking at? On the 737 pic, you can see 3 items on the right side, and 2 on the left. One on each side measures angle of attack, whilst the others are for the air data systems. These are called pitot probes, and are forward facing, and project out into the airflow. To measure airspeed, a second probe system is needed (generally close to the rear of the aircraft) called a static port - these don't project into the airflow. Basically, your airspeed is measured by the difference between the pitot pressure and the static pressure. Altitude is measured by the difference between the static pressure and a known datum pressure.
So, what happens if we block any of the probes?
The pitot probe has both an inlet and outlet (drain). If both parts are blocked (perhaps by icing), then the pressure inside the probe will remain constant. If you climb it will show an airspeed increase (because the actual pressure as measured by the static port is reducing), and it will show a decrease if you descend. If you remain at the same level, it will show a constant airspeed even if you change the actual speed. If only the inlet side is blocked, the pressure will drop to the static pressure, and the indicated airspeed will fall, eventually falling to zero.
If the static port is blocked, then it will hold the last pressure it had. Altitude indications will not change with an actual climb or descent. Airspeed will over-read during descent, and under-read during a climb.
If a mud wasp blocks the pitot during your time on the ground, then during the take off roll the airspeed will not increase (if totally blocked)....so cause for an abort.
Causes...icing, even though all of these probes have deicing systems (i.e. heating). Volcanic ash. Insects. Bird strike damage. Covers being left in place.
Aircraft can be successfully flown with any of these issues....they should almost be a non event, especially the icing ones, which will often cure themselves. Angle of attack data can actually be used instead of airspeed, although it isn't directly displayed in most aircraft. But, even though the aircraft should be quite flyable, blockages within the pitot/static systems have been implicated in many accidents, across all brands.
