"If the pitot tube becomes blocked or malfunctions and there is no airspeed indicator, how can one judge the speed of the airplane?" - Philip
“Airspeed indicator malfunctions can come in a variety of ways – sometimes no indication, other times exceedingly high or low indications. In any case, you need to be familiar with the pitch attitudes and power settings that give you various airspeeds.
For example, if you have climb power on and the nose is maybe just below the horizon, I’ll bet you have approximately climb speed.
You can improve your ability to fly without indicated airspeed by climbing to altitude and practicing glides with various flap settings and noting the pitch attitudes. Remember the picture out the window and now practice the same glides with the airspeed indicator covered. These same attitudes will work when you are in the pattern for landing.
Also when practicing stalls, note the pitch attitude when you are near stall at various power and flap settings. Remember these attitudes and avoid them if your airspeed indicator is not working.
Another tool if installed could be a GPS ground speed, but remember this is ground speed only so you need to account for any headwind or tailwind.
Learning to fly the aircraft by looking out the window and paying attention to the pitch attitude and power is the way to master the plane. Then the instruments simply become tools to further refine our flying.”