A probably stupid question from a non-pilot

[Mike-Bracknell]

Originally Posted by fernytickles

Thats assuming the automatic deployment system worked. Back up the back up systems ad infinitum.....

Consider this...

You've got 3 pitots giving constant readings that the computers check against each other for a disagree, like so:

A=400,400,400,400,400,453,750,896,345
B=400,400,400,400,400,400,603,342,533
C=400,400,400,400,400,400,400,400,400
X=-,-,-,-,-,-,400,400,400

where X is the standby pitot. As you can see, after the 5th iteration pitot A gives a differing reading, and after the 6th iteration both A & B pitots are giving differing readings. The computer is constantly monitoring the differences between pitots A,B,&C, and throws a 'disagree' at either iteration 5 or 6. This is where X is deployed and the computer can then take it's reading and compare it against each of the other pitots one by one to see if you find an 'agree' since the probability of 2 or more pitots agreeing is infinitely smaller than the probability of pitots disagreeing.

Further still, if all 3 pitots (A,B,C) aren't producing what they were (400kts) at X deployment, you can state that X is 'probably' correct and the other 3 pitots are iced, by looking at the trace back prior to the disagree and comparing it with X values.

If it then disagreed, you could retract it, clean it, redeploy it, and check again.

It can't be worse than blighting a cockpit with 3 disagreeing forms of information with no way to recover from it, can it?

[Jumbo Driver]

Originally Posted by Slasher

However, risks are always there for blocked pitots (I had one struck by lightning once) but we're trained to deal with it.

Mmmmm ... I started professional flying in 1970 but I don't recall ever seeing a "Pitot Struck by Lightning" Check List ...

JD

[RainingLogic]

Now now Jumbo, are you questioning the veracity of his statement, or impuning his qualifications? I'll have you know that I have personally had my landing gear blown off my lightning, my copilot killed by lightning, my Jepps were burned up right in my lap by lightning and had to shoot an instrument approach by memory. God Damn Lightning! Your just gonna have to take my word it!

[west lakes]

We've got lots (and lots) of wind-turbines around here and they us something I think is "l@ser-radar" to work out the windspeed to adapt the windmill to the wind condition.

If you look closely at most of them they have an anemometer on the rear of the generator housing, there is also wind direction monitoring there.

(just an aside)

[cwatters]

LIDAR is starting to be used by wind farm developers as a possible alternative to a temporary anemometer mast. I gather a key advantage of LIDAR is that unlike a mast it doesn't need Planning Permission (UK).

[Slasher]

but I don't recall ever seeing a "Pitot Struck by Lightning" Check List ...

There isn't any, just as there isn't any to cover every possible
situation one might encounter. Suffice to say the huge bright
loud and sphincter-loosening BANG gave some clue as to why
I suddenly lost my ASI and a couple of other items (B737-2)
Pitots and lightning bolts don't get on well socially.

[Jumbo Driver]

Originally Posted by Slasher

There isn't any, just as there isn't any to cover every possible situation one might encounter. Suffice to say the huge bright loud and sphincter-loosening BANG gave some clue as to why I suddenly lost my ASI and a couple of other items (B737-2) Pitots and lightning bolts don't get on well socially.

B737-2 ... ? Aaahh, yes - I recall we often had one or two of those doing circuits in our Aft Cargo Hold on Ultra Long Haul sectors ...


JD

[Lemain]

..why does flying safely rely on something that is obviously liable to blackages or damage?

Further to the other answers already posted, you might wonder why something as crude as a pitot is still used. Well, the KISS principle is valid but what makes the pitot so spectacularly the right choice for the job is that it responds not just to the SPEED through the air but the density of the air. Consider a speedboat pitot (a common way to log the speed of a fast water craft). Going through water, by the time you get to ten knots or so there is quite a good pressure and that can indicate speed. Imagine watching that boat's speedo with the boat out of the water and a ten knot wind....obviously and intuitively you know that that it won't read anything like that speed. Similarly, in outer space it will read zero. What causes the 'lift' to keep the aircraft in the air depends not just on the airspeed but also the density of the air. The rarer the atmosphere the higher the airspeed needed to keep it flying. As we go higher, the atmospheric pressure drops (which is why we use a barometer to measure altitude) and the air becomes less dense, giving less lift. An understandable technical explanation can be seen here: Aerospaceweb.org | Ask Us - Lift Equation

The pitot responds to speed and air density. A GPS gives you groundspeed, not speed through the air: when flying there is always wind - anywhere from the nose, side or tail, and the pitot looks forward, which is what we're really looking for as the shape of the surfaces are designed to provide lift with airspeed from the nose of the aircraft. An anemometer (cups) doesn't really respond to density. Gyros don't do it (already explained) and Inertial Navigation (INS) is really just a different way of providing what GPS now provides at lower cost and greater accuracy.

I once stuck the pitot back on my aeroplane with five-minute epoxy. The engineer had changed the tyres one day, and gone home. He'd obviously knocked the pitot as it was hanging by its tubing and no other engineer was around. Down the hardware store for some epoxy and I was rolling half an hour later. Dead simple technology. Note: Children, don't try this at home.

[G&T ice n slice]

Thanks for that, I REALLY hadn't actually thought it all through! the altitude/density issue never occurred to me.

Oh well, I'll have to try & think of some other mousetrap to invent a better one of!

Cheers

G.