lexkid12300
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Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 12:58 am

Hey guys,

I was on flightaware tonight and noticed something strange about a certain plane's flight path, and wanted to see what you guys think. Here's the link to flightaware, it's flight 438 on 19-Oct-2011.
http://flightaware.com/live/flight/MPE438

It's a Canadian North Boeing 737-200 on a 3 hour flight. When i look at the "flight path" diagram they give, i can see that the plane keeps deviating from its flight path left and right as if there's no autopilot keeping it going straight. Here's a few pictures of the flight path with differing zooms:



And here is a portion of the flight log; you can see that the heading changes by a LOT every minute, yet the speed and altitude seem to hold just fine:
http://i52.tinypic.com/1z169fp.jpg

And it's not just the zoom that's making this flight path seem so eratic, i just checked several other 737 flights and some paths are VERY straight while others are just like the one above. A 737-200 with first Air has a path similar to the one above, while another 737-200 with Sierra Pacific Airlines has a very nice and straight flight path, shown here:


And here's the path of a 737-300 with Southwest Airlines, very true and straight:


And here's the flight log for this Southwest 737-300 flight. You can see the heading doesn't change more than a degree each minute:
http://i55.tinypic.com/ivxytj.jpg

And for comparison, here's the flight path of a DC-9-10 with Kalitta charters, again nice and straight:


Does anyone have an idea as to why a few of those 737-200 flight paths are so "wavy"? Is it a lack of autopilot? And if so, why haven't they been installed?? It's 2011 already!

I think it's interesting that the flight path of that 737-200 with Sierra Pacific was straight but the others were not... I also checked the flight paths of other planes flying in the same areas as the First Air and Canadian North flights and their flight paths are perfect- so it can't just be the winds. I also checked a DC-3 flight (Buffalo Airways) and its flight path is VERY wavy- but i would expect this since i can't imagine its autopilot is very good... being 75 years old i mean!

Thanks for the input!
 
tb727
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 1:26 am

Yes they do and it looks wavy probably because of the location of where the flight track was taken, not what the airplane was physically doing. Paths over Canada look very wavy, I'm not sure of the exact reason but it probably has to do with where they are getting the track information, the real reason is probably buried somewhere on flightaware. Flightaware doesn't necessarily represent reality.

The way I look at it is there are a lot of overlapping radar sites in the US so you are going to get a better look at what the airplane is doing from more angles giving a smoother, more accurate line.
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MrChips
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 1:27 am

There are a number of simple explanations for what you see, each of which could account for some or all of the answer:

1) Flightaware maps use a form of Mercator projection, which produces a lot of distortion at high latitudes. What looks like a huge deviation might turn out to be no larger than any of your other examples.

2) Their flight plan routes can be pretty inaccurate in my experience, especially when dealing with areas like northern Canada.

3) While I can't comment specifically on that particular aircraft's equipment level, they might be following an NDB airway, as it is possible (but unlikely these days) that particular aircraft isn't equipped with a certified GPS. Following an NDB airway, especially over long distances, isn't as precise as flying with more modern means.

These are by no means the only possibilities; only the first ones that came to the top of my mind. If I was going to bet on the most plausible explanation, I'd go with #1.
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lexkid12300
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 1:45 am

Ah good points!

But it still doesn't explain why some of other flights going in and out of CYFB and CYOW have much straighter flight paths.

I thought flightaware got its data from the FAA/similar organization? And don't they get their data from the plane's transponder? You'd think the plane would be sending out its heading, altitude, etc via the transponder. Maybe each plane is using a different method for calculating its heading, one method being more accurate than the other.
 
tdscanuck
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 2:28 am

Quoting lexkid12300 (Reply 3):

But it still doesn't explain why some of other flights going in and out of CYFB and CYOW have much straighter flight paths.

Other aircraft are almost certainly using some composite of GPS/inertial navigation, which works a lot better.

Quoting lexkid12300 (Reply 3):

I thought flightaware got its data from the FAA/similar organization?

They do (not the FAA in Canada, but the equivalent).

Quoting lexkid12300 (Reply 3):
And don't they get their data from the plane's transponder? You'd think the plane would be sending out its heading, altitude, etc via the transponder.

Transponders on aircraft that old only report altitude. Position still comes from conventional ATC radar.

Tom.
 
lexkid12300
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 2:33 am

So it safe to say that the autopilot on this 737-200 might not have the plane's correct position, resulting in the deviations? In this case flightaware might actually be accurate- and that the plane's autopilot keeps getting lost!
 
tdscanuck
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 4:02 am

Quoting lexkid12300 (Reply 5):
So it safe to say that the autopilot on this 737-200 might not have the plane's correct position, resulting in the deviations?

No flight management computer knows the plane's true position, there is always a margin of error. What's more likely with an old autopilot is a low gain coupled with a fairly loose tolerance...it "wanders" between a wider allowable path. There's no point in having an autopilot with really high gain try to track a path 0.01nm wide when the position is only accurate to, say, 0.5nm.

Tom.
 
Fabo
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 9:11 am

The 737-200 normally came with two kinds of autopilot.

One variant is a Sperry SP-77 autopilot, with controls similar to the older 707 and 727 style APs. I am not sure, but I consider it possible, that navigation sources in these aircraft are not coupled to the autopilot directly and pilots have to input heading via the rotary bug or via bank selectors.

Later, Sperry SP-177 was installed, that is as far as I know roughly equivalent to autopilots on (early?) 737 classics.
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HAWK21M
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 10:03 am

http://www.b737.org.uk/images/glare-100-v2.jpg
Sperry SP-77

http://www.b737.org.uk/images/glare-200adv.jpg
Sperry SP-177
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Starlionblue
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 10:45 am

Quoting HAWK21M (Reply 8):
Sperry SP-77

Now that's badass steam gauge tech right there.

Thx for pics MEL!
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YokoTsuno
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 11:15 am

Quoting tdscanuck (Reply 6):
What's more likely with an old autopilot is a low gain coupled with a fairly loose tolerance...
I don't really understand the term "Gain" here. Does this have the same meaning as Gain or 1/Proportional band in control systems?
 
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longhauler
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 3:07 pm

All of the B737-200s I flew had the Sperry SP-77 autopilot shown above. As those aircraft are now at Canadian North, then the autopilot likely has not changed.

Heading was maintained using a rotary bug on the Captain's HSI. Oddly enough, the F/O's HSI was not so equipped, and a rough form of control wheel steering was used. The only exception to this was when coupled to a VOR radial, (something we never did as it was pretty rough) or coupled to an ILS approach, which could be flown to minima.

However, on the aircraft we often had other sources of nav information. Some aircraft had ONS, one had INS and later GPS was installed. While the track error was displayed, the actual tracking was still done using the heading bug if the Captain were flying, or CWS is the F/O were flying. In other words the autopilot was not coupled to the ONS, INS of GPS.

Looking at the plot above on the YFB-YOW flight, the initial heading look like there may be an error with Flightaware. Heading swings like 360 to 190 to 360 to 190 to 360 to 190 all within 2 minutes would be a hell of a ride if accurate. However, the later parts of the flight look accurate, then it appears the pilots were "bracketing" the course offered by the GPS, but were using either the heading bug or CWS to maintain it.
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tdscanuck
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RE: Does The 737-200 Have An Autopilot?

Thu Oct 20, 2011 5:18 pm

Quoting YokoTsuno (Reply 10):
I don't really understand the term "Gain" here. Does this have the same meaning as Gain or 1/Proportional band in control systems?

Same meaning...how much you multiply the error to get the actuator command. A high gain system will generally track more tightly and quickly but be more subject to instability.

Tom.
 
Fabo
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RE: Does The 737-200 Have An Autopilot?

Fri Oct 21, 2011 9:46 am

Quoting longhauler (Reply 11):
While the track error was displayed, the actual tracking was still done using the heading bug if the Captain were flying, or CWS is the F/O were flying. In other words the autopilot was not coupled to the ONS, INS of GPS.

Thank you for confirming that.

Also, there is a great webpage with many information on b737. I am not sure if this can be counted as advertisment, I hope not (I am in no way affiliated with the page) - www.b737.org.uk
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26point2
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RE: Does The 737-200 Have An Autopilot?

Sat Oct 22, 2011 2:49 pm

This aircraft is in RVSM a/s which, among other things, requires use of an autopilot with altitude hold.
 
Fly2HMO
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RE: Does The 737-200 Have An Autopilot?

Sat Oct 22, 2011 11:31 pm

Quoting lexkid12300 (Thread starter):
Here's a few pictures of the flight path with differing zooms:

May I recommend next time you save the pictures in .png format. They are much better for preserving smooth graphics and texts than JPEG and dont produce the same annoying artifacts that JPEG does.
 
speedbird128
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RE: Does The 737-200 Have An Autopilot?

Sun Oct 23, 2011 3:17 pm

Quoting lexkid12300 (Thread starter):
And here's the path of a 737-300 with Southwest Airlines, very true and straight

Can a B733 make F390? AFAIK they are only good for F370.
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Fabo
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RE: Does The 737-200 Have An Autopilot?

Sun Oct 23, 2011 3:31 pm

Quoting Speedbird128 (Reply 16):
Can a B733 make F390? AFAIK they are only good for F370.

That one is indeed a Boeing 737-700 (ICAO code B737, IATA 73G or B73G if I am not mistaken)
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DIJKKIJK
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RE: Does The 737-200 Have An Autopilot?

Sun Oct 23, 2011 5:46 pm

When even older birds like the 707, DC-8 and the Constellation had autopilots, surely the 737-200 must have em as well  
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HAWK21M
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RE: Does The 737-200 Have An Autopilot?

Mon Oct 24, 2011 11:31 am

Quoting Fabo (Reply 13):
there is a great webpage with many information on b737

Is sure is a good website on the B737 with excellent notes & pics.I've posted a few of my notes on it too.

About the Autopilot....I remember the common problem encountered on the SP-77 was the Pitch and Roll channel not engaging or tripping in flight.
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