By Geoff Farr - 30th August 2012 7:03am
For The sake of a nail the shoe was lost
For the sake of a shoe the horse was lost
For the sake of a horse the battle was lost
For the sake of a battle the Kingdom was lost
I have noticed for some time that our airspeed indicator (ASI) is a bit slow to register as we gather speed during the take off run.
Now! The ASI registers its airspeed, by air being forced into an open tube which is suspended below the wing. This tube (about 5 millimetres diameter) is connected to the ASI by a small plastic tube.
The increasing air pressure from the forward motion of the vehicle pushes the needle round the dial of the ASI thus telling the pilot when the airspeed is sufficient take off and leave the ground.
When landing, and as the vehicle slows, the ASI will show diminishing speed and advise the pilot when the wing is ready to give up its burden and allow the aircraft to settle upon the ground.
It therefore follows that this should only occur when the machine is very close to the ground....say about 12 inches (30 centimetres). If this diminishing speed should occur at say 5000 feet above the ground, the wing and therefore the aircraft will stall and begin to fall.
You may begin to see therefore that the function of the ASI is quite important to the well being of pilot and passengers. It further follows that the pilot must be taught how and in what circumstances the airspeed must be managed so as to create a stall and also to recover from a stall should one occur accidently.
The ASI, on large and sophisticated aeroplanes as well as small aeroplanes, uses the same principle of a small open tube suspended on the outside of the craft and in the direct airflow to push an increasing or decreasing volume of air along the linking tube to the rear of the ASI. This pressure pushes the speed needle around the dial for the pilot to see.
Sophisticated aircraft have automatic pilots to manage these things and so save the pilot from a lot of work. We, however do not aspire to such luxuries and are obliged to manage our airspeed through the control stick at all times.
Where an automatic pilot is employed precautions have to be taken to ensure that nothing blocks this tube and so it is heated so as to prevent water getting in to the tube and freezing. If something like this should occur the ASI will tell lies to the automatic pilot with rather bad results.
We do not want ever to see the ASI tell lies whether the pilot is a clever machine or me. So, back we come to our rather sluggish ASI.
Whilst the aircraft is hangared, a cover is sometimes placed over the open tube to prevent the ingress of anything nasty which might block the tube.
I don't favour a cover myself for I have seen occasions when the cover is not removed during the pre-flight inspection and during the subsequent flight the ASI failed to register.
However, since nothing is perfect, having no cover can lead to the tube becoming blocked or partially blocked by "things and stuff."
To take note of the condition of the open tube is one of the functions of the pre-flight inspection. But the human eye cannot see around corners or beyond the mouth of the tube.
So......after landing Saturday afternoon I resolved to check the tube for partial blockage.
There is only one way to do this and that involves taking out the whole instrument panel and disconnecting the plastic tube from the back of the ASI then blowing back it in the reverse direction to the normal flow.
This operation is not particularly sophisticated, but it does need two people. One stationed at each end of the pipe.
I employed my son Stephen to take station at the outer end of the tube whist I positioned myself at the instrument end and prepared to blow.
Now, Stephen has had this job before and his duties in the matter consist of observing any object that emerges from the pipe and relaying that information to the blower.
Having on a previous occasion received a face full of all manner of rubbish, he resolved to put that experience to good use. Using appropriate thought and forward planning and knowing how important his observations must be, he secured a clear plastic bag over the open end of the tube to contain any evidence which may appear.
After the first lung full he reported to me that he had the culprit captured in his plastic bag. We had between us ejected a small spider who I surmise had been resident in the tube for several weeks.
We have no record of what the very alive spider said on being evicted from his hitherto comfortable home. But he doesn't live there any more and our ASI is not now sluggish in operation for which we are grateful.
This leads me to muse on the journeys undertaken by our lodger and to what heights he has aspired during his illegal occupation. The wind must have whistled past his ears on many occasions.
I am grateful that he/she did not find a mate to share his abode or the resultant eggs would undoubtedly have caused a complete blockage of our precious pipe.
As I wrote at the beginning of this piece " For the sake of a spider an aircraft was lost."
Well, not this time!
Monday 21st Oct
Meet at the Bridge Inn
Scout and Guide Hall