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Airspeed Indicator Question  
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Posted (6 years 11 months 4 weeks 12 hours ago) and read 2396 times:

Regarding airspeed indicators -- why can't they read reliably below 60 to 80 knots?

And how do they determine groundspeed if the speed-gauge isn't reliable below 60-80 knots. I was told with the 747 for example 20-25 knots was as fast as you could taxi with using full-steering... or something like that -- either way how do they determine growndspeed?


Andrea Kent

12 replies: All unread, jump to last
 
User currently offlineHAL From United States of America, joined Jan 2002, 2568 posts, RR: 53
Reply 1, posted (6 years 11 months 4 weeks 12 hours ago) and read 2391 times:



Quoting Blackbird (Thread starter):
either way how do they determine growndspeed?

Today, we all use GPS groundspeed as our most reliable indicator for taxiing. It's shown somewhere on almost every primary flight display or Nav display. Even in our old round-guage DC-10's we could pull up the info on the FMS computer. In the old days??? I was still puttering around in 2-seat Cessnas so I don't know.

HAL



One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 2, posted (6 years 11 months 4 weeks 12 hours ago) and read 2389 times:



Quoting Blackbird (Thread starter):
I was told with the 747 for example 20-25 knots was as fast as you could taxi with using full-steering... or something like that -- either way how do they determine growndspeed?

10 Knots for a 90 degree turn. On the 400 there is a ground speed readout on the ND. On the classic, you select TK/GS on the INS and there you go!

The ground speed is computed via the IRS/INS, just as it would be in flight.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 3, posted (6 years 11 months 4 weeks 12 hours ago) and read 2384 times:



Quoting Blackbird (Thread starter):
Regarding airspeed indicators -- why can't they read reliably below 60 to 80 knots?

And how do they determine groundspeed if the speed-gauge isn't reliable below 60-80 knots. I was told with the 747 for example 20-25 knots was as fast as you could taxi with using full-steering... or something like that -- either way how do they determine growndspeed?

Airspeed below 60 knots is not useful information on an airliner.  Smile Anyway, the ASI cannot read groundspeed, because of wind effects.

The 747 classic had a groundspeed readout based on INS velocity available in digital form on the HSI. The dedicated TAS indicator stops reading below about 115 knots. The 744 of course has similar data available on EFIS.

I think 10 knots is regarded as a better speed for full steering angle on a 747. Above 20 and you will start to get scuffing vibration.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (6 years 11 months 4 weeks 9 hours ago) and read 2357 times:



Quoting Blackbird (Thread starter):
either way how do they determine growndspeed?

There is also a speed sensor on the wheels. It's used to drive the autospoilers and antiskid, among other things.

Tom.


User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 5, posted (6 years 11 months 4 weeks 8 hours ago) and read 2352 times:



Quoting Blackbird (Thread starter):
Regarding airspeed indicators -- why can't they read reliably below 60 to 80 knots?

ASIs work by measuring a pressure differential between static and dynamic flow pressure.
Like everything else 'pressure' related (e.g. lift, drag etc.) this differential doesn't vary linearly with speed but is a function of speed squared.
So at low airspeeds ASIs become disproportionately more inaccurate as the pressure differential measured becomes relatively tiny.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 6, posted (6 years 11 months 4 weeks 8 hours ago) and read 2346 times:



Quoting Blackbird (Thread starter):
Regarding airspeed indicators -- why can't they read reliably below 60 to 80 knots?

Well they do in some aircraft...  Wink [like many of the ones I fly...]

I guess the pitot tube just can't transfer enough pitot below certain airspeeds Big grin



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offline3MilesToWRO From Poland, joined Mar 2006, 281 posts, RR: 0
Reply 7, posted (6 years 11 months 4 weeks 7 hours ago) and read 2328 times:



Quoting Blackbird (Thread starter):
Regarding airspeed indicators -- why can't they read reliably below 60 to 80 knots?

Every measuring device has its range of valid values and it must not necessarily begin from 0. Actually I think hardly ever it does. Your car's speed indicator also doesn't start exactly from 0, does it? (Well, if if does it's only pretending  Wink )
If you need reliable values of speeds in flight, you need a device that works well in range, say, 100 to 500 knots (for an airliner, I mean). What's less or more is not very useful (or not so important) so the design of speedometer can be less accurate (up to total uselessness) for it.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (6 years 11 months 3 weeks 6 days 21 hours ago) and read 2266 times:

You are moving a mechanical needle through air pressure. The air pressure is roughly proportional to the airspeed squared. At 60 knots it is one quarter of the pressure at 120 knots. At some point or other the friction etc in the mechanics will become significant enough, compared to the air pressure, for the resulting errors to be large enough to render the reading inaccurate to the point where it is effectively useless. At that point, insert peg in instrument face.

As for finding a nice, suitable taxi speed there are two TSMDs, taxi speed monitoring devices, in most commercial aircraft. They operate mainly by measuring the changes of angles between visually recognizable features in the aircraft surroundings, and are typically able to give a somewhat rough but exact enough estimate of actual speed over ground for taxi. These are installed in TSMDHs, taxi speed monitoring device holders, in the cockpit in order to give the devices a relatively unobstructed view of the surroundings. The TSMDs are unfortunately prone to wear and must be replaced every eight hours or so, at least. To facilitate quick installation and removal the TSMDs are usually mounted to the TSMDHs through four-point belts equipped with quick-release buckles. They are almost always combined with the control actuators. They are very sensitive devices, and great care is taken to keep their environment within the specified operating range. Most people working in aviation agree that they are the single biggest source of trouble. Still, few would like to do without them.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offline777236ER From , joined Dec 1969, posts, RR:
Reply 9, posted (6 years 11 months 3 weeks 6 days 18 hours ago) and read 2190 times:



Quoting FredT (Reply 8):
As for finding a nice, suitable taxi speed there are two TSMDs, taxi speed monitoring devices, in most commercial aircraft. They operate mainly by measuring the changes of angles between visually recognizable features in the aircraft surroundings, and are typically able to give a somewhat rough but exact enough estimate of actual speed over ground for taxi. These are installed in TSMDHs, taxi speed monitoring device holders, in the cockpit in order to give the devices a relatively unobstructed view of the surroundings. The TSMDs are unfortunately prone to wear and must be replaced every eight hours or so, at least. To facilitate quick installation and removal the TSMDs are usually mounted to the TSMDHs through four-point belts equipped with quick-release buckles. They are almost always combined with the control actuators. They are very sensitive devices, and great care is taken to keep their environment within the specified operating range. Most people working in aviation agree that they are the single biggest source of trouble. Still, few would like to do without them.

Did you know that TSMDs cause more than half of all crashes? Amazing, but true.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 10, posted (6 years 11 months 3 weeks 6 days 16 hours ago) and read 2154 times:



Quoting Tdscanuck (Reply 4):
There is also a speed sensor on the wheels. It's used to drive the autospoilers and antiskid, among other things.

True, but no speed readout is provided to the crew from this source.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offline737tdi From United States of America, joined Sep 2007, 923 posts, RR: 2
Reply 11, posted (6 years 11 months 3 weeks 6 days 16 hours ago) and read 2148 times:
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Had a write-up the other day. Control wheel feels stiff (just so happened it was the night of the Cowboy/Packer football game). Amazingly enough after removal of the TSMD/F.....'d up stick actuator, the aircraft was inspected and returned to service with no faults found. Go figure. Have to say though, this is very rare with our flight crews.

737tdi


User currently offlineTornado82 From , joined Dec 1969, posts, RR:
Reply 12, posted (6 years 11 months 3 weeks 4 days 19 hours ago) and read 2062 times:



Quoting Jetlagged (Reply 3):
Anyway, the ASI cannot read groundspeed, because of wind effects.

Yep, the new G1000 equipped light piston singles are probably the best examples of this because of the better resolution of their ASI's. Talking to a gentleman with a "glass" C172 while sitting on the ground with the electrics turned on to show me his new toy... the thing was dancing around up to 28kts or so on a windy day. Theoretically, had we been taxing on a paralllel to a runway so that the wind was a tailwind, we could have gone 28kts GS and the ASI would read 0.


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