Air Speed Indicators

[liseczek]

Maybe I'm being naive. There have been so many deaths (including AF447) due to blocked/iced up pitot tubes. IMHO it would be so simple to drop down a propellor (in exactlythe same way as a RAT (Ram Air Turbine)) and count the propellor blades as they go past with a Hall Effect Transducer to give an airspeed reading. This is exactly the way cruise control works on cars. The hub could be oil filled to give resistance and prevent freewheeling. I am sure this would give extremely accurate readings if calibrated, and the logic to make this the primary airspeed indicator in case of failed pitots would be child's play. The pitot is one step on from the spring loaded vane found on early aircraft, and I have flown a Tiger Moth with exacly that arrangement - a little spring loaded paddle with a pointer moving across a scale to show the airspeed. Even a variation of that simple mechanism would make a great emergency ASI, and I'm sure would have saved hundreds of lives.
Birgenair Flight 301, Aeroperu Flight 603 are two more which come to mind.

All the engineering effort seems to go into better and better pitot tubes, and makes the assumption that if one ices up, the others won't, and even when only one ices up, the pilot can GUESS which is giving the correct reading.. :ugh:

Sometimes simplest is best. When the space program started up, the Americans spent millions designing a ballpoint pen which would write in zero-g. The Russians use a pencil. 'Nuff said?

[Fargoo]

Generally moving parts are a lot more unreliable and prone to failure and or errors.

You can't get more simple than the good old pitot tube hence its continued use in even the most modern of aircraft.

[liseczek]

That is the whole philosophy of the RAT. I'm not saying to scrap pitots, but to have a backup which is not prone to icing, spiders, wasps etc. By your logic you would scrap the RAT because it's complex and prone to failure? The RAT is also a last resort, and not designed to take the place of the aircraft systems.

[Mad (Flt) Scientist]

A RAT type arrangement would also be prone to icing, and its a lot harder to anti-ice a rotating component. But, it could be done

A RAT type arrangement would be prone to errors at both ends of the speed range (many RATs have to shed load at low speed or have high speed limits)

But, biggest issue - you have to KNOW you have a problem in order to revert to a backup. Devising a monitoring scheme that can determine when ALL of the standard pitots have gone bad and revert to a single backup is a very challenging task. To the extent that today we tend to rely on the most complex computing on the aircraft to make that determination - the pilot. If he can't decide today to go into the unreliable airspeeds checklist, how is he going to be any better at determining he needs to deploy the "Speed RAT"?

Don't get me wrong, dissimilar speed sensing would be nice. But the dissimilar system has to be broadly as good as the redundant pitots it would be displacing. So far no-one seems to have such an option.

[Gulfstreamaviator]

There are many ways of getting a ball-park airspeed.

However the ambient conditions are needed to give a precise figure.

I am no engineer, but umple pillock.

If there is a discrepency between GPS airspeed, and ADC (pitot) drive), then perhaps a error CAS message, and a displayed value.

Perhaps the No 3 system should be from a NON ADC source, just like in old days.....

Or lets call it No 4.

Glf

[flame_bringer]

Thats why the probes anti icing system came up and it's a no go MEL item in many cases, so as to overcome the icing drawback, and moving properller would make for a hell a lot of weight which is not very good for the weight saving phillosophy that the manufacturers aim for, let alone servicing them and the potential defects that might occure in them, (broken propeller due to debris going onto them, vibration and blades wieght unbalance ,bird strike ...etc).:rolleyes:

[Fargoo]

Unreliable?
That is the whole philosophy of the RAT. I'm not saying to scrap pitots, but to have a backup which is not prone to icing, spiders, wasps etc. By your logic you would scrap the RAT because it's complex and prone to failure? The RAT is also a last resort, and not designed to take the place of the aircraft systems.

I don't really get where the RAT comes into this? They are deployed to do the task of either a hydraulic pump or electrical generator both of which are complex machines with many moving parts.

There are normally 3 pitot probes with the 3rd being the standby.

A pitot probe has no moving parts and is inherently more reliable than something which would have many moving parts. That's why I don't agree with your idea.

If you're proposing a 4th RAT type system then I'd have to ask - where do you stop. If you have backups for every backup then quickly your aircraft becomes too heavy to leave the ground.

[liseczek]

I am simply comparing the philosophy of a drop down propellor as an emergency ASI in the same way as the RAT is a drop-down. Might as well scrap the RAT since by your logic it is prone to icing and a broken propellor? RATs have been deployed many times in civilian and military aircraft, and I haven't heard of one icing up or breaking the propellor.

The 'emergency ASI' doesn'tneed to be over-engineered. Any of the 3 accidents I mentioned above (and there have been many more) would have been avoided had the pilots been able to drop down the emergency ASI and had been given even a rough indication, such as 200kts, airspeed increasing or decreasing. Any pilot would know what to do in such circumstances.

As for 2 active and one standby ASI, it's like the old joke about sheltering under a tree in a downpour. When this one gets wet, I'll shelter under another one. :ugh:

I repeat, hundreds of lives have been lost because of blocked pitots on aircraft flown by pilots with many many thousands of hours experience, and the implication you are making is that they could easily have recovered by using the GPS/INS or whatever, and were simply incompetent?

When 2 pitots are giving conflicting readings, the wonderful on-board computers effectively say 'we don't know what's going on' and disconnect the autopilot, leaving it up to the pilot to guess which ASI is correct (assuming only one is blocked). What is needed is a final arbiter, i.e. the propellor driven drop-down ASI. As for calibration, you do realise that even a pitot has to be calibrated, and the air pressure converted to a reading. Double the airspeed, there is 4 time more air pressure, Treble the speed, and it becomes 9 times greater. The propellor driven one would be easier to calibrate since it would be more linear, double the speed is double the revs. OK, some cleverclogs will go on about propellor slip and resistance not being linear, but this is easy enough to test and calibrate. The crucial range needs to be around 150 - 300 knots, and an oil-filled centre bearing with vanes to give some resistance and prevent freewheeling as the airspeed decreases,

There is a certain mindset in engineering which is roughly: this is the way our fathers did it and their grandfathers before them, so it is the right way.

[Fargoo]

There is a certain mindset in engineering which is roughly: this is the way our fathers did it and their grandfathers before them, so it is the right way.

That is the most ludicrous statement I've ever seen posted here. I'm afraid your idea isn't a good one but since you don't want to listen i'm out :ugh:

I'm still open mouthed that someone would suggest the aircraft engineering community isn't progressive :eek:

[dClbydalpha]

Would a low friction propellor read True or Indicated Airspeed?

My instinct is to say that it would be TAS, and therefore you need a source of air density in order to calculate the necessary EAS/IAS for the display.

[Alber Ratman]

Dynamic Pitots, Static ports and OAT probes are a solution that are simple and effective 99.99999999999999999 % of the time. 447 crashed because the crew were incapable of flying the bird correctly in the situation they put themselves into, with the icing problems that affected the pitot system (and cleared less than a minute later after the A/P had cut out).

[lugs4744]

When 2 pitots are giving conflicting readings, the wonderful on-board computers effectively say 'we don't know what's going on' and disconnect the autopilot, leaving it up to the pilot to guess which ASI is correct (assuming only one is blocked).

I think you'll find your wrong with that statement, whilst both probes feed their individual instruments only one system(usually the co-pilot/#2 system feeds the computers, for the very reason you gave!
Yes, you would get a disrepency between the instruments but the autopilot will not suffer the same problem. Autopilot systems are designed with redundancy i.e they wont disconnect after one failure. The autopilot will WARN the pilot if it detects a discrepency between what the pitot system is telling it and what other systems have calculated based on gps/ins/irs systems.

I see your point for using a rat type system as a back up, but as others have pointed out, why install a backup to a system that is equally as vunerable to icing, if not more.Due to it's size even the slightest ice on the blades of the rat will change it's aerodynamic properties and thus give you an incorrect indication,ice on the blades will make them heavier therefore introducing inertia, square one succesfully relocated!

A better option and one that is already used, is to have an emergency x-feed to the computer from deep within the systems, when the computer feeding probe fails or becomes blocked the clean free system then feeds the computer.

[winglit]

Autopilot systems are designed with redundancy i.e they wont disconnect after one failure

I wish that was true on the B767 I have AOG at the moment.

All 3 A/Ps disconnected and failed to re-engage. Turns out to be a faulty SAM, giving a false aileron lockout, go figure!

My grandad used to have a windmill on the top of his shed roof. It was attached to a little man who looked like he was cranking it. Maybe we could bolt one of those on the nose. The pilot can count the "bobs" and calculate how fast he is going!

Death by pitot indeed! Air France was death by stupid pilots.

[lugs4744]

I wish that was true on the B767 I have AOG at the moment.

All 3 A/Ps disconnected and failed to re-engage. Turns out to be a faulty SAM, giving a false aileron lockout, go figure!



Sound like fun,Headaches all round with that one!

Someone made the point earlier about having a limit to backups and they are spot on.
Overall the pitot systems on aircraft have good records and most failures can be traced back to poor maintenance.After all blockages come about because the covers haven't been kept on when in for maintenance not because some flying insect has flew down the tube whilst taxiing!