wardialer
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IRS Ground Speed Question

Thu Mar 23, 2006 12:27 am

In the picture above, you can see the Ground Speed on the IRS screen. My question is, during a take-off roll, does that ground speed indicator have a "lag" to it during take-off roll?

In other words, when the take-off roll begins, do the numbers switch really really fast like this: 123456789 and so on.....or, since the IRS gatheres the ground speed information from a satellite, do the numbers switch or "lag" like this: 1 3 8 10 18 23 36 47 59

So, what I mean, I have this handheld GPS unit, it measures the Ground Speed very slowly when I accelerate...So, it would start its ground speed measurements like this...

1 and then after a few seconds later....25.....then a few seconds later....32...then a few seconds later.....44...then a few seconds later....55...and so on....

So, on an aircraft like in the picture where it shows "GS" does it measure the same way as my GPS unit that I have or, does it measure the speed (during take-off roll) more faster like this: 123456789 and so on????

Please help on this...I would appreciate it.


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Photo © Guy Daems - Brussels Aviation Photography



ALso, one more question.

The GS indicator in the picture above, does that measure the speed the same way as we see the speedometer meter in our cars?? Is this the same?

Thanks
 
corey07850
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 12:59 am

Quoting Wardialer (Thread starter):
since the IRS gatheres the ground speed information from a satellite

Well an IRS doesn't use satellites for information, which is a key advantage of the system. It uses internal acclerometers to judge changes in acceleration in the X and Y plane, this information is then calculated to give distance, speed, etc...

To answer your main question, I'm not sure how the numbers change as I've never flown anything that sophisticated  Wink
 
wardialer
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 1:03 am

Well, can someone please be able to answer this please...?

I really wish to know this.
 
PhilSquares
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 1:40 am

There is a "lag" on the G/S. The rate of the lag is directly proportional to the increase in speed, that is the faster the acceleration, the more the lag is.

The IRS don't use GPS. It computes the G/S based on the rate of acceleration. The display you see is the FMS computed G/S. That will use all the IRS units and the GPS to compute a speed.
Fly fast, live slow
 
Okie
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 2:26 am

Quoting Wardialer (Thread starter):
do the numbers switch or "lag" like this: 1 3 8 10 18 23 36 47 59

The numbers lag in relationship to processor updates to the display depending on how many milliseconds the manufacturer chooses. Some display a snap shot, some with trend averaging from the previous number or numbers to buffer the characters displayed from jumping up and down between scans.

The problem is the human eye and brain can not process the information that computers operate. Anything changing over about 24 frames per second the eye and the brain can not see/process precisely. (go to a movie, you never see the screen go black between the 24 times per second that the new frame is projected, or your monitor either, your eyes and brain average the snap shot to show movement)

The display will be behind but not in the sense that you are referring. Only by milliseconds and how often the manufacture chooses to update the display. Now I can not answer for aircraft but in industrial applications anything updating to a human display more often than .25 seconds and usually .50 seconds just causes trouble for humans to process the information/numbers that are changing quickly while in the mean time all the of the rest of the processes/caluations are going at the processor scan speed.

Example: In a case that the information received from the accelerometer is 5kts/second from a standing start and your screen is updated four times per second then in that 1 second time frame then at least one sequential number will be left out on the display. (Simplified because we actually have 0,1,2,3,4,5 and a do not know at what point the update started)

Okie
 
aogdesk
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 2:52 am

Quoting PhilSquares (Reply 3):
There is a "lag" on the G/S.

I'm actually surprised to see that there is latency in the groundspeed readout, but then again, whenever I'm taxiing something, my concern as a mechanic isn't keeping an eye on groundspeed, I'm sure Phil is much more in tune with the intracacies of the flight deck. The accelerometers used in a/c navigation essentially know where they are, and they know when they've been moved from where they started. Picture sitting in your car while its stopped, you're holding up a string with a rubber ball on the end, its hanging completely vertical. Now step hard on the gas (watch for oncoming traffic and elderly ladies) and what happens? The ball/string reacts immediately, shoots backward, and the higher the rate of acceleration the farther the ball goes. Thats a basic accelerometer. IF there were a groundspeed readout attached to that ball and string, (you'd have an ungodly mess of wires in your car) it would react just as immediately. Perhaps theres a reason for the latency of the g/s readout that Philsquares refers to.
 
David L
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 2:58 am

Quoting Aogdesk (Reply 5):
Perhaps theres a reason for the latency of the g/s readout that Philsquares refers to.

Anything to do with the processor speed in the FMS? I get the impression they're really slow compared to the average PC.
 
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Starlionblue
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 3:11 am

Quoting Okie (Reply 4):
The problem is the human eye and brain can not process the information that computers operate. Anything changing over about 24 frames per second the eye and the brain can not see/process precisely. (go to a movie, you never see the screen go black between the 24 times per second that the new frame is projected, or your monitor either, your eyes and brain average the snap shot to show movement)



Quoting David L (Reply 6):
Quoting Aogdesk (Reply 5):
Perhaps theres a reason for the latency of the g/s readout that Philsquares refers to.

Anything to do with the processor speed in the FMS? I get the impression they're really slow compared to the average PC.

Sure. Most of those processors are four or five generations behind the bleeding edge. The best of 1990 if you will  Wink


As Okie says, information more often than 24 times a second is pretty useless anyway, with some exceptions:
- The eye/brain can actually "see" things that appears for only 1/100 of a second. Pretty useful for a fighter pilot when finding enemy planes, but not really on instruments.
- CRT computer monitors update about 70-85/second. Any slower and they appear to flicker. LCDs update down to 60 times a second but since LCD pixels are either on or off (as opposed to CRTs that are lit, then fade) there is no flicker.


I would also venture that aircraft ground speed doesn't need to be updated more than a few times a second since that is not how the information is used. Who needs the ground speed indicator to tell them the plane is accelerating?  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
wardialer
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 3:19 am

So, you guys are telling me that there is indeed a lag when the aircraft accelerates? right? So do the numbers skip during a take-off roll? Meaning that there is a skip in the numbering sequence?

I wish there was a video during a take-off roll that would just zoom right in on that FMS screen to see what happens...

So during a taxi, I would assume that the numbers are sequenced like this: 1, 2, 3, 4 knots....But, the faster the aircraft goes (like a take-off roll) the numbers would go like this: 1, 26, 39, 48, 66, 69, 77 knots...Am I right or wrong???

The slower the aircraft goes like a ground taxi....then the numbers or the measurements would not lag, instead, they would go in order like 1 2 3 4 5 6 7...

I forgot to mention this that I do have this Lufthansa flight in the cockpit video and there was shot of this FMS speed in flight and I did see the "GS" speed of 471 knots and then like after a few sconds it would hop from 471 to 472 then a slit second back to 471 then again after a slit second it hops again back to 472...

So my understanding would be this:

As the aircraft starts to settle its speed once reaching cruising altitiude, then there is no "lag"....the numbers then tend to change in seconds rather then a few seconds after...Or they tend to change in 1 knot increments.....

But, if the aircraft starts to make erratic or sudden speed changes (like a take-off roll) then yes, the speed can be measured in 5 knots or even 12 knots increments per maybe like 5 seconds or so...

Am I right?
 
David L
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 3:34 am

Quoting Starlionblue (Reply 7):
As Okie says, information more often than 24 times a second is pretty useless anyway

Agreed and that would explain the display skipping numbers but not a general lag, e.g. once every 25 frames the speed would be correct rather than once every 25 frames you're shown what the ground speed was a second ago.

Quoting Starlionblue (Reply 7):
I would also venture that aircraft ground speed doesn't need to be updated more than a few times a second since that is not how the information is used.

Of course... unless you're approaching the tire speed limit, in which case you've probably got a lot more to worry about.  Smile

It's not important but I'm still mildly intrigued.
 
PhilSquares
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 3:50 am

Quoting Aogdesk (Reply 5):
Perhaps there's a reason for the latency of the g/s readout that Philsquares refers to

A couple of things need to be clarified. First of all the G/S and wind vector, really don't mean much on take off roll. Sure when you are taxiing with the airspeed indicator at 0 it's a good way to monitor your taxi speed.

However, on the takeoff roll, what we really look is the airspeed tape as it comes up to speed. All the V speeds are based on indicated air speed not GND speed. Also ground speed is displayed from first movement. However it's not until the takeoff roll, anound 100 KIAS the wind vector will appear, assuming there is wind. So now you have the mathematical model of the IRS (3) providing their inputs to the FMC and now the wind component is then added to equation. Oh and don't for get the GPS has it's say, plus the nav-nav based updating. All those factors do take some time. At cruise it's not a big deal since the variations are small.

But realize the G/S display isn't really used for anything. With the possibility of an about you'd have the exact G/S the brakes were applied. Now you have a good estimate of cooling time.
Fly fast, live slow
 
wardialer
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 3:51 am

Would guys agree to my theory that I posted above??? Please see my last post.
 
aogdesk
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 4:56 am

Wardialer, I'd say that you're probably mostly correct. Once again, in a car, if you've got a non-analog speed display, as you accelerate quickly, you're not going to see 1-2-3-4-5-6-7-8-9 all the way up to 70mph, you'll probably see 8-15-22-29-36....something like that......

The LH video where you see the g/s oscillating between single digits is most likely because the FMS is computing at 471.9 or thereabouts and its bouncing between that and 472.

While I'm not certain how your handheld GPS actually receives its data, I'm reasonably certain saying that it doesn't receive a constant flow of information that would allow it to update speed transparently to you. More than likely, it polls (sends out a request for data) and does that at a preset interval, maybe once every five seconds, maybe once very 1.5 seconds, I have no idea. But for a consumer grade GPS, the need for constantly updated info isn't there.
 
Okie
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 5:19 am

Quoting Wardialer (Reply 8):
As the aircraft starts to settle its speed once reaching cruising altitiude, then there is no "lag"....the numbers then tend to change in seconds rather then a few seconds after...Or they tend to change in 1 knot increments.....

Close, but you keep using the term "lag" which technically is incorrect but I think you have the idea. It is basically the amount of change between the previous update to the display at whatever rate/timeframe the manufacturer chooses to update the display again.(unfortunately we do not have that information on the updates)

Quoting Wardialer (Reply 8):
The slower the aircraft goes like a ground taxi....then the numbers or the measurements would not lag, instead, they would go in order like 1 2 3 4 5 6 7...

There is that "lag" word again.

Here try this: I am taking buffets, wind changes, processor averaging out of the example using acceleration only.
If an airplane accelerates at 5 knots per second say to 250 knots and the display updates 5 times per second then you would see each and every number on the display up to 250 knots. If the display updates 2.5 times per second then you would see every other number. If the display updated once per second then you would see every 5 th number on the display.

Okie
 
wardialer
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 12:17 pm

OK my fault...I just did not know how to phrase this question...It even confused me to be honest.

So, I got your other last idea in the last post. Its more understandable. But the problem is, we do not know how long it updates..Thats the problem.

Giving me a rough guess here. What do you think the rate of update is on these FMS ground speed indicators?

Will it update 5 times per second, 2 time per second, or, it depands how fast the RATE-OF-ACCELERATION is...??

Thanks for the clear up on this...I just did not know how to phrase this question.
 
A/c train
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 3:05 pm

An IRS uses accelerometers to measure accelerations in different attitudes, it integrates the accleration once for distance and twice for speed, there are 2 integrations carried out in an IRS.
 
bphendri
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 3:39 pm

Quoting PhilSquares (Reply 10):
But realize the G/S display isn't really used for anything

Huh? Isn't ground speed very usefull for dead reckoning your position? Certianly you would not plot your course using TAS.
 
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Jetlagged
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 5:06 pm

Quoting Bphendri (Reply 16):
Huh? Isn't ground speed very usefull for dead reckoning your position? Certianly you would not plot your course using TAS.

You have IRS and GPS feeding data to the FMS and you're going to use ground speed for dead reckoning  Confused
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
PhilSquares
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 5:51 pm

Quoting Bphendri (Reply 16):
Huh? Isn't ground speed very usefull for dead reckoning your position? Certianly you would not plot your course using TAS.

If you're down to DR, you have much bigger problems than plotting your course to worry about!

That would mean you have no FMC (2 installed) no IRS (3 installed) no VOR/DME (installed) no FMS CDU (3 installed). Time to land if you ask me!!!
Fly fast, live slow
 
PhilSquares
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 5:51 pm

Quoting Bphendri (Reply 16):
Huh? Isn't ground speed very usefull for dead reckoning your position? Certianly you would not plot your course using TAS.

If you're down to DR, you have much bigger problems than plotting your course to worry about!

That would mean you have no FMC (2 installed) no IRS (3 installed) no VOR/DME (2 installed) no FMS CDU (3 installed). Time to land if you ask me!!!
Fly fast, live slow
 
A/c train
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RE: IRS Ground Speed Question

Thu Mar 23, 2006 7:06 pm

Dead reckoning?? old manual methods ! almost prehistoric!
 
bphendri
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 12:35 am

Quoting PhilSquares (Reply 19):
That would mean you have no FMC (2 installed) no IRS (3 installed) no VOR/DME (2 installed) no FMS CDU (3 installed). Time to land if you ask me!!!

Thats right, and thats exactly what you would use your DR plot to attempt to do.

As my dad allways told me when teaching me to sail, The day your too lazy to follow time honored procedure is the day time honored procedure will bite you in the butt.
 
David L
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 12:41 am

Quoting Bphendri (Reply 21):
As my dad allways told me when teaching me to sail, The day your too lazy to follow time honored procedure is the day time honored procedure will bite you in the butt.

But you have a lot more time for DR when sailing. In any case, if the problem was serious enough to take out the IRS, GPS and radio navigation, I'd have thought the groundspeed indication would be lost, too.  Smile
 
wardialer
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 1:01 am

All I want to know is that the ground speed indicator in the picture above, does it update its speed similar to my handheld GPS meaning that does it update like every 15 seconds like my GPS, or...does it updaye much faster than that?
 
PhilSquares
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 1:03 am

Quoting Bphendri (Reply 21):
Thats right, and thats exactly what you would use your DR plot to attempt to do.

As my dad allways told me when teaching me to sail, The day your too lazy to follow time honored procedure is the day time honored procedure will bite you in the butt.

As a sailor, I can assure you it's very different in a 744 or any glass cockpit aircraft. There has never been a case, that I know of, where a modern day glass aircraft has lost it's ability to navigate using the IRS/FMC. You're talking about 5 completely separate systems. Personally, I'd be on the ground long before I had to rely on DR.

Not to burst your bubble, but that's called judgement.
Fly fast, live slow
 
David L
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 1:34 am

Quoting Wardialer (Reply 23):
All I want to know is that the ground speed indicator in the picture above, does it update its speed similar to my handheld GPS meaning that does it update like every 15 seconds like my GPS, or...does it updaye much faster than that?

I suspect the problem may be that no-one watches it during take-off. During climb, cruise and approach, the speed may be changing too slowly to tell whether it's updating every fraction of a second or every few seconds.

What about during deceleration on the runway, e.g. watching for 80 knots, or whatever, to disengage reverse thrust (or is that airspeed, too?)? Even then one might be focussing too much on what the speed is to notice how often it's updated. We need a volunteer!
 
wardialer
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 2:59 am

I'll tell you what folks...

Why dont we archive this post or thread and lets see if there are any "glass cockpit" A320 or Boeing 747-400 pilots who will watch it the next time they fly??

If any pilots are reading this...please watch the FMS Ground Speed indicator during take-off...Please, and then post back to see what happened.
 
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ManuCH
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 6:39 am

This is just a guess: I think the GS indicator is always updated at a predefined rate (say, once every 3 seconds). It doesn't depend how quickly you're accelerating, it's just a fixed value the manufacturer decided based on several factors. It wouldn't make sense to update a digital indicator every 1/10th of a second, you wouldn't be fast enough to read it anyway if it updated too quickly.

The numbers don't "lag" (ie. they're not "behind" the actual GS), they only refresh slowly. But you get the point.

On a good old (well, not so old) general aviation Garmin GPS which indicates GS, it is updated at always the same rate (which I don't remember).

-Manuel
Never trust a statistic you didn't fake yourself
 
wardialer
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 9:27 am

If it updates every 3 seconds...then would it skip increments like this?

3, 6, 9. 12, and so on...?

The ONLY ONLY ONLY way to tell or answer this thread, is to actually sit in the jump seat in one of the glass panel type aircrafts like the A320...or something..

Thats the only way.

If someone can have a video of this, or maybe a pilot can tell us the nextime he or she flies, than would be great for all of us here.
 
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ManuCH
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RE: IRS Ground Speed Question

Fri Mar 24, 2006 6:20 pm

Quoting Wardialer (Reply 28):
If it updates every 3 seconds...then would it skip increments like this?
3, 6, 9. 12, and so on...?

Well, if the plane is in a constant acceleration (1 kt/s in this case), then yes. It will more probably be something like 3, 6, 12, 20, 30, 42, ... whatever, the exact numbers depend on how fast the plane is going *duh*  Smile

Quoting Wardialer (Reply 28):
The ONLY ONLY ONLY way to tell or answer this thread, is to actually sit in the jump seat in one of the glass panel type aircrafts like the A320...or something..

I suppose so.
Never trust a statistic you didn't fake yourself
 
wardialer
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RE: IRS Ground Speed Question

Sat Mar 25, 2006 12:15 am

Like I said, have a pilot come on here and post back on what he saw...
 
wardialer
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RE: IRS Ground Speed Question

Mon May 15, 2006 2:13 am

I just thought of a great scenario by using a Calculator to explain on what I mean...I know its really really really silly....Just please read on what I mean by this whole entire thread please...On my question and the picture on my first post above, I asked whether if that Ground Speed (where it shows "GS452") on the FMS on that FMS screen, on how the ground speed updates during a take-off roll, and I just thought of a good but rather silly scenario...Here you go....

1. Open the Calculator in Windows and in the keypad press the (+) button (on the NUMERIC portion of the keyboard and then press 1....

2. Then....(on your numerical key-pad again HOLD DOWN the ENTER button....You'll notice how really really fast the numbers move rapidly up....Right???

Now, my question is....Is that how FAST that Ground Speed (in the photo) updates during a take-off roll or does it move slower or depending on how fast the aircraft accellerates down the runway? Or, when you just TAP the ENTER key fast instead of holding it down, is that more like it when an aircraft accellerates down a runway?

Please advise and I would appreciate it...Thanks.

[Edited 2006-05-14 19:18:01]

[Edited 2006-05-14 19:19:57]

[Edited 2006-05-14 19:21:03]

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