A probably stupid question from a non-pilot
 

If I've understood everything correctly, the cause of several mishaps has related to blocked pitot tubes.

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

can't they be replaced by gyroscopes, grouped in threes mutually at 90 degrees
i.e. 1 set front-back, 1 set left-right and 1 set up-down?

I gather that solid-state laser gyros are terribly reliable? wouldn't this work?

As I said, probably a stupid question.... sorry...

Gyros are used for navigation equipment and sense the speed over the ground; it is the speed of the air over the wing that affects the lift of an aircraft and all relevant flying speeds are based on the relative speed of the air hence the need for a pitot tube.

because Gyros cannot measure your speed relative to surrounding air.... which is what keeps you flying.

Gyros will only measure your speed through space.

"Gyros will only measure your speed through space"


Erm, nope, they don't even do that.

Aircraft Inertial Navigation System
 

The aircraft knows where it is at all times. It knows this because it knows where it isn’t. By subtracting where it is from where it isn’t, or from where it isn’t from where it is (depending on which is greater), it obtains a difference or deviation.


The inertial guidance system uses deviations to generate corrective commands to drive the aircraft from a position where it is to a position where it isn’t. The aircraft arrives at the position where it wasn’t; consequently the position where it was is now the position where it isn’t.


In the event that the position where it is now is not the same position where it originally wasn’t, the system acquires a variation, this variation being the difference between where the aircraft is and where the aircraft wasn’t. If the variation is considered to be a significant factor, it too may be corrected by the inertial guidance system. However, the aircraft must now know, also, where it was.


The “thought process” of the aircraft is as follows: because a variation has modified some of the information the aircraft obtained, the aircraft is not sure where it was. It now subtracts where it should be from where it wasn’t, and by differentiating this from the algebraic difference between its deviation and it’s variation, it obtains the difference called error, usually attributed to the pilot. :ok:

So behave all the aircrafts in the land of Oz.
Except for the aircrafts of the Esmerald City that navigate using a subtly different process : they only consider as a valid variation the difference between where the aircraft wasn’t and where the aircraft is and not the difference between where the aircraft is and where the aircraft wasn’t.

Luc

G&T

An aircraft flies due to the difference between the speed of the aircraft and the speed of the wind.

The simplest and most reliable method of doing this is a pitot.

A perfect inertial nav system (which doesn't exist, incidentally) would still be useless if it had no way of knowing the speed of the air around it.

ie, the air would also have to be dotted with inertial nav systems in constant radio comunication with the aircrafts inertial nav system to make this an effective method of working out airspeed.

This being the question (and not your presumed answer)

My answer is that

Flying safely does not rely on something that is obviously liable to blackages or damage?

The failure condition is anticipated and safe aircraft operation is assured by virtue of backup systems and pilot training. Most of the problems occur (unsafe flying) when the pilot becomes confused and fails to employ the backup systems and operating technique.

I'm afraid that even adding additional systems would still not assure safe flying if the pilot didn't understand what was happening and failed to use the backup system correctly.

OP: More then a few pilots don't have the where with all to cross check instruments. A famous Dallas airliner accident with a blocked pitot tube comes to mind.

Ah, thanks for the replies. I hadn't thought it thru enough had I? I was thinking along the lines of the 'speed measurement' factor giving the answers, but of course the wind is also moving.

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.

could the same not work on an aircraft?

Cheers

G

Rate gyros measure rotational rate. Accelerometers measure acceleration, both in 'inertial space'. To measure air speed need something in addition, ie directly 'sampling' the air flow or doing a calculation if you also have independent measure of ground speed.

This ?

Anemometer - Wikipedia, the free encyclopedia

Wings care little about ground speed or which way is up. They work only by an appropriate amount of air flowing over them arriving at a sensible angle. To measure the speed you need to sample the air arriving from the front - a pitot tube. The difference between the air pressure in the pitot tube and the static air pressure is termed 'airspeed'. You could measure the same thing by generating a suction (like a venturi) but this is even more prone to errors as any surface contamination (bugs, dead birds, ice, tape etc.) would lead to a suction which would no longer be proportional to airspeed. Good question though.

PM

Seen some documents that say the 787's will have a Vsyn(synthetic) function with the loss of airspeed indicators. Technically I think it said loss of Vcas is replaced by Vsyn.

G&T ice n slice,
a number of instrument manufacturers are experimenting LIDAR systems as relative wind back-up measurement instruments.
The matter is uneasy because the scattering process has to be different in lower atmosphere (IR on aerosols) and in upper atmosphere (UV on ozone or air molecules) or in clear sky versus in visible moisture. A reliable system could end up requiring up to four different LIDARs operating parallely.
There is still a long way before these devices attain a level of reliability equivalent to pitot tubes.

Luc

some links on those experiments:
NEw Standby Lidar InstrumEnt - Transport - Research & Innovation - European Commission
Michigan Aerospace Corporation - Molecular Optical Air Data System (MOADS)
Optical air data systems
Science Links Japan | Airborne Coherent Doppler Lidar-Flight Test Results

The FBW Airbus types have combined Air Data Computer and Inertial Reference Systems, but the speed indicated to the pilots is effectively Calibrated Air Speed derived from the pressute data from the pitots. The very latest ones come with the BUSS (Back Up Speed Scale) where is the ADRs fail or there is a major discrepancy in the indication, the can indicate a speed and altitude reference which is derived from the GPS. I've tried it in the simulator and it works very well.

Not a dumb question G&T.

Pitot and static tubes rely on ambient air to gather info to present to the airspeed indicator, altimeter and vertical speed indicator. Blocked pitots (either through human error or Nature) are a fact of life.

Pitots and static air pressure sources (which old school drivers like myself call "weatherheads") in something as unwieldly complicated as a bloody 320 are sent to computers that poke their fingers into places you wouldn't even begin to fathom - thus if these computers get the wrong data all sorts of bells and whistles go off for subsystems that are actually working perfectly. And yes its intially bloody confusing when it happens. That's why some of us do our preflight walkarounds checking the weatherheads as a separate inspection from the rest of the aircraft, esp when its been in the hangar for maintenance or first flight of the day.

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

With the BUSS, only the altitude is derived from GPS data.
The speed is derived from incidence probes.
Luc

Thanks for the replies chaps.

I still think there should be a backup pitot that's deployable from behind a flap in the case of a disagree. Granted it wouldn't be failsafe, but it would be relatively immune from gaffer tape and ice accumulation up until the point of deployment (which might give you the survivability you need, especially if deployed automatically on a disagree - it'd also give the pilots an easily decipherable error message to base any troubleshooting on)

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?

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

JD
;)

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! ;)

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)

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).

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
:)

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.

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.