Gyro Upgrade:
The SigmaTek Bootstrap Directional Gyro
After our 30-year old gyro began experiencing fits while under IFR, we decided it was time to replace it with a new unit. If you're in the market for a new DG or are installing one as part of a larger installation, this article may be of some help to you.
The Why and What
Given that we had previous experience with Sigma-Tek gyros (both attitude and directional) and had found them to be exceptionally reliable, we decided to go for another Sigma-Tek unit.
Based on my research and a lot of help from my avionics shop, I found that Sigma Tek makes three basic types of DG's:
Standard gyro with "dummy" heading reminder ($1000)
This provides a heading bug, but nothing else. The gyro will not interface with anything except the vacuum lines.
Believe it or not, a heading bug is a very useful tool, and I have never considered it a crutch. Whether you're flying in a crew environment or single-pilot IFR, this really sharpens one's heading control because it gives you something to reference as you go about your various tasks.
If you fly IFR and don't have any plans to install an autopilot or other sophisticated avionics this might be the best (if not the least expensive) choice.
Gyro with autopilot heading output ($1300)
Models with an "autopilot" interface provide heading output compatible with selected "simple" autopilots that can track a pre-established course such as the S-TEC model 20, 30, 40, or 50.
When an System 50 is in HDG (heading) mode, for example, the bug will function as you'd expect. Turn the bug to a heading up to 180 degrees in either direction and the autopilot will turn the airplane in the direction of the bug until the current heading is that selected with the bug.
When it comes to course tracking/interception (NAV mode), however, this bug is only accurate to a few degrees of 12 o'clock (the current heading). You must first establish the airplane on the heading that will hold the course, put the bug on the current heading, and then enable "NAV" mode in order for the autopilot to track the course. If you don't put the bug on the appropriate heading within approximately +/- 5 degrees, in my experience, the autopilot will not track the course properly.
The DG's autopilot interface is manufacturer-specific, so you must order the model to match the autopilot you have installed (or, in our case, will eventually install).
Gyro with bootstrap heading output ($1700)
This provides a heading reference for autopilots that that can intercept a course up to 45 degrees either side of the current heading, such as the S-TEC 60 (with which I have personal experience) or the new 55X, or heading-dependent devices such as a Sandel or Stormscope.
The one downside to this unit is that it needs an external 400Hz 26 Volt AC supply to excite the bootstrap interface. These are about $400. And, yes, this is ancient analog interface technology, but everything in avionics moves at a snails pace, so I'm sure we'll be using this until GA gets a reasonably-priced full glass cockpit based on a digital AHRS.
Choices, choices
With this information, we began to talk about the future plans for the airplane. My friend said he definitely wants to install an S-TEC autopilot at some point, and he really likes the Sandel EHSI he had installed in his 182. I mentioned that we might get more utility for less cost if we instead invested in a WX-500 remote mounted Stormscope and displayed the information on the Garmin 430.
We couldn't possibly make a decision about which unit to install, but we did know that either unit would require an external heading reference and we knew we didn't want to invest the considerable sum to install a remote gyro and flux gate. For an extra $400, the bootstrap DG seemed like a no-brainer, so we chose that option. While I thought it ludicrous for SigmaTek to build a bootstrap gyro without a built-in power supply, in reality it allowed us to defer that cost until we perform the installation that requires the bootstrap interface.
Upsides and Downsides
If you're looking for a disadvantage to a bootstrap DG, it's simple -- like any other standard DG, it isn't corrected to the Earth's magnetic field via a flux gate, so if the Sandel were driven by the DG we'd have to reset it to correct for precession just like we would any "non-slaved" HSI. This is a small price to pay, in my opinion, to save at least $10K in parts and labor, as well as the maintenance and overhaul costs associated with this complex system.
On the upside, we feel the DG is actually preferable to the remote gyro since it could be used as a backup in case the projector bulb in the Sandel failed in flight, which would otherwise necessitate a no-gyro approach.
To Light or Not to Light
Incidentally, SigmaTek also provides internal lighting for all gyro models for approximately $150 extra, but since the long-term plan for this aircraft called for a complete rework of the panel and the use of instrument lighting "wedges", we felt internal lighting would actually be a nuisance, as we expect it would be nearly impossible to match the lighting intensity and color temperature across the instruments.