Compass swing on an RV6

[Sam Buchanan]

Hmmm...how shall I respond?? I will behave myself and withhold from a snarky response and simply say:
An Air Swing is one of the five "approved" methods for a compass swing per AC 43-215 and is a viable alternative to the traditional method described above as requested per the OP.
From my experience an air swing is far easier and faster to accomplish. Kinda falls in the work smarter not harder category

Please educate me.

The compass needs to indicate the direction the nose of the aircraft is pointed. If airborne, the GPS may be indicating a ground track of 360 but due to a crosswind the nose of the aircraft is pointed several degrees east of ground track. How do you use the GPS in this situation to calibrate the compass?

[Airmutt]

The procedure does require relatively smooth air.
Only have to fly the eight cardinal and ordinal points. (N, NE, E, SE, S, SW, W, NW)
The GPS or the INS (OK, probably not a reality for most of us) is your heading source. Think of it as a master compass.

Remember this is an exercise in recording the compass deviation and making corrections as necessary. Don’t think of it in terms of flying a track from Point A to B, because you’re not. You’re simply flying a constant GPS heading. It’s surprisingly simple.

Fly the GPS heading long enough to allow the mag compass to stabilize and record the deviation. In a perfect world your DG, mag compass and GPS should all read the same.

[Sam Buchanan]

The procedure does require relatively smooth air.
Only have to fly the eight cardinal and ordinal points. (N, NE, E, SE, S, SW, W, NW)
The GPS or the INS (OK, probably not a reality for most of us) is your heading source. Think of it as a master compass.

Remember this is an exercise in recording the compass deviation and making corrections as necessary. Don’t think of it in terms of flying a track from Point A to B, because you’re not. You’re simply flying a constant GPS heading. It’s surprisingly simple.

Bear with me, I'm still not seeing this.

Let's say I decide to fly a constant GPS heading of 360. The only "heading" GPS knows is ground track. So the plane is traveling in a track that is due north. However, due to a crosswind, the nose has to be pointed into a crab to accommodate the wind and maintain the 360 ground track. So the compass isn't aimed North. How do I determine which direction the compass (aircraft nose) is pointed and how much it is deviating from where it is pointed?

The only way to hold a particular heading with GPS is to fly that particular ground track, the GPS doesn't know which way the wind is blowing.

Fly the GPS heading long enough to allow the mag compass to stabilize and record the deviation. In a perfect world your DG, mag compass and GPS should all read the same.

The only perfect world where this would exist is in one with absolutely no crosswind.

I read AC 43-215 and even though it states a GPS can be used for an air swing, the procedure calls for aligning the aircraft to the 30* incremental headings using the aircraft's directional gyro.

quote
Note: The 30-degree increments may be determined by aligning an aircraft gyro with north, as indicated by the compensated standby compass, and then using the gyro to indicate the increments.
end quote

Now that procedure I can comprehend, but the GPS thing doesn't make sense to me because GPS and a directional gyro are two completely different methods of determining direction, one being based on ground track and influenced by wind the other on the aircraft's heading with no wind influence.

Here is the document:

https://www.faa.gov/documentLibrary/.../AC-43-215.pdf

[rwanttaja]

I'm having a little trouble with this, too. Let's assume a plane is flying north (as indicated by his compass) and there's a strong wind from the west.

The wind displaces the airplane to the east, vs. a straight-north heading. The GPS has no idea what the heading of the airplane is, only its ground track. So it says you're actually flying ~15 degrees. You can't use that 15 degrees to determine how much compensation the compass needs.

Mind you, all you'd need is a calm day. If the plane cruises a 150 knots and there's a 15 knot wind, the error introduced is about five degrees. But maybe that's OK. Note that this is about the accuracy the AC expects ("Starting on any convenient 30-degree heading, align the aircraft so that thereference compass system reads within 5 degrees of the desired heading.")

Ron "Isn't this California?" Wanttaja

[Airmutt]

Let’s try this one more time…
The GPS receiver natively reads in true north, but can calculate magnetic north based on its true position and data tables. However, in this case the unit is NOT navigating it is merely being used to convert true heading and display it as magnetic heading. This is a steady heading maneuver, not an exercise in navigation.
And as a side bar, the DG in the 172 I rent is slaved to the GPS. Back to football.

[rwanttaja]

As ever, when my mind turns to various items in the Federal Air Regulations (excuse me, "14CFR Part 91"), my thoughts turn in one direction: Loop holes.

First, of course, is the very regulation that "requires" me to have a compass (and, of course, have it properly calibrated): 14CFR 91.205.

But 14CFR 91.205 is titled, "Powered civil aircraft with standard category U. S. airworthiness certificates; instrument and equipment requirements."

And I don't HAVE a powered civil aircraft with a standard category certificate. My Fly Baby is licensed in the Special category.

"Ah-HA!" the pendants pounce. "Your operating limitations require the aircraft to be equipped per 91.205!!!!!"

Uhhhh....no, they don't.

My Operating Limitations state, "This aircraft shall contain the placards, markings, etc. as required by FAR 91.31."

Now, flip through your copy of Part 91 and tell me: What does 91.31 say?

Well, nothing. Its listed under "91.27-91.99 [Reserved]".

So, Mr. FAA Man: Show me this 91.31 I'm supposed to comply with.....

Now, I KNOW what happened. In the ~40 years since my plane was licensed, the FAA re-ordered the regs in Part 91. Maybe 91.31 was identical to 91.205.

And maybe it wasn't. My operating limits that "The aircraft shall contain the placards, markings, etc. as required by FAR 91.31." It doesn't say the aircraft must contain the placards, markings, AND INSTRUMENTS as required by FAR 91.31." All it talks about is the placards and markings.

Instruments are an AWFUL big category to include as an "et cetera." Note that the current 91.205, which DOES address instruments, doesn't say a word about placards and markings.

So, Mr. FAA Man: Show me this 91.31 I'm supposed to comply with. See, I've got al the placards and markings....what were you saying about instruments?

[And yes, I know the current version of Operating Limitations requires 91.205 equipment...]

Ron Wanttaja

[rwanttaja]

Let’s try this one more time…
The GPS receiver natively reads in true north, but can calculate magnetic north based on its true position and data tables. However, in this case the unit is NOT navigating it is merely being used to convert true heading and display it as magnetic heading.

Mmmm.... SOME GPS receivers. Mine sure can't.

Ron Wanttaja

[Sam Buchanan]

Let’s try this one more time…
The GPS receiver natively reads in true north, but can calculate magnetic north based on its true position and data tables. However, in this case the unit is NOT navigating it is merely being used to convert true heading and display it as magnetic heading. This is a steady heading maneuver, not an exercise in navigation.
And as a side bar, the DG in the 172 I rent is slaved to the GPS. Back to football.

You still haven't addressed the difference in what GPS and a compass indicate when wind is a factor.

Yes, a GPS could be used to calibrate a compass if there was no wind, but how often do we see that occur? The referenced circular isn't written very well because it recommends "smooth" air. The air aloft can feel "smooth" to the aviator and still be blowing 40 kts. The only way a GPS could be used for swinging a compass is in calm air.

The Boy Scout compass on the ramp seems to be a MUCH easier method and doesn't require perfect weather.

[Dana]

The procedure does require relatively smooth air.
Only have to fly the eight cardinal and ordinal points. (N, NE, E, SE, S, SW, W, NW)
The GPS or the INS (OK, probably not a reality for most of us) is your heading source. Think of it as a master compass.

Let’s try this one more time…
The GPS receiver natively reads in true north, but can calculate magnetic north based on its true position and data tables. However, in this case the unit is NOT navigating it is merely being used to convert true heading and display it as magnetic heading.

Uh, no. GPS provides NO heading information, unless the unit also happens to include a magnetic sensor. GPS provides only position; from two positions it can determine track (not heading) and groundspeed (not airspeed). Only in dead calm, or a direct headwind or tailwind, will the track be equal to the heading.

Now it is possible to determine the actual headings and airspeed from GPS tracks in several different directions at a constant airspeed in absolutely steady winds, but it requires some algebraic calculations; it's not something you're going to do in flight.

But don't feel bad; whoever wrote that AC doesn't understand it either.

[Airmutt]

Yes, GPS in its pure sense does not know or provide heading.

Hand held units take advantage of MEMS and contain devices such as accelerometers, magnetometers or GNSS antennas all of which provide heading information. Almost every Garmin handheld has a compass page and the user can define the format - True, Magnetic, Grid. Hence their ability to display a compass rose.

Fixed units take advantage of similar devices (typically standalone) or receive heading info for other units such as an INS/INUs. The communication with these units is prioritized.

We did inflight alignments but we were equipped with dual GPS and INU units. In reality it’s a blended solution. Perhaps it was a bad use of terminology on my part but as an Atlanta native I call any soda a Coke.

I understand there are methods to determine heading by GPS lone but it involves using two units or two antennas. Unfortunately the science is beyond me. If I remember correctly it’s based on two points establish a line. By tracking the movement of the line the heading can be established? Someone way smarter than me can confirm or deny.