Vance Harral

Most of these analog gauges are D'Arsonval devices, in which a needle is attached to a delicate spring and suspended between poles of an electromagnet. DC current through the electromagnet twists the needle against the spring in proportion to the thing being measured. It's possible your needle has just "slipped" relative to the spring, such that zero current causes it to point to the 400 mark, and nonzero current pegs it. If that's the case, maybe it could be adjusted by someone qualified to open up the gauge. But the more common failure mode is when something allows much more current to flow through the gauge than designed. This pegs the needle hard and bends the spring. If that's what happened, the spring needs to be replaced, and it's often more cost effective just to replace the whole gauge. Equally important, you have to figure out what caused the damaging event before installing a new gauge. In this case, that involves the CHT probe circuit. The factory CHT probe in vintage Mooneys is a thermistor (not a thermocouple), which is a device whose resistance changes with temperature. Changing resistance changes the current through the gauge, thereby moving the needle. It's possible for the thermistor probe (or the wiring to it) to fail in such a way that a short circuit develops, i.e. zero-ish resistance. If that happens, a lot of current will flow through the circuit, and this can damage the gauge as discussed above. Because of this, it's really important not to just install a new gauge without debugging the CHT probe circuit, as you could wind up just damaging the new gauge in the same way the old one was damaged. You can check all this stuff if you're handy with electronics. Disconnect the gauge from the rest of the circuit. Measure the resistance through the probe (and wiring to it) to see if there is a short. If so, fix that first (repair broken wiring, install new CHT probe, whatever). Then you can test the gauge independently with a current source if you're inclined. You might be able to fix it, but probably easier just to source a used one from someone who has discarded their factory gauges in favor of a modern engine monitor.

Not that one more data point is going to change anyone's mind, but here's another apparent ham-fisted idiot. Posted just now in the Vintage Mooney facebook group.

"All other things being equal"... This is the part I'm not getting. Are you saying the pressure drop dh/dl is independent of the filter media area? As in, there is the same pressure drop from the front to the back side of the filter regardless of whether the filter media has zero surface area (i.e. there is no filter), or it's an infinitely folded pleat? If that's actually the case, then yeah, it's just linear math. But I don't think that's how it works. I'm open to enlightenment. That's my point. You can't change the design of the filter - including its total surface area - without changing the drop dh/dl. Stacking two filters on top of each other most certainly does change the total surface area of filter material that the air passes through, but you're arguing that doesn't count because of the pressure drop it causes. I'm still trying to understand why the design of a single sheet (of however many pleats) doesn't also cause a variation in pressure drop.

Sure. But the stock size filter is not necessarily "more than adequate for the needs of the engine", for practical reasons. To give one example, the original M20 design used a Continental C-145 engine, whose intake path nicely matched the position of the "chin" airframe intake on the cowl, and the size of that intake presumably provided plenty of air to the engine. When the design was retrofitted with a Lycoming O-360, the airfame intake was no longer aligned with the engine intake, and this was resolved with an air box that makes the air effectively traverse an S-turn, reducing induction efficiency. The Lycoming also needed slightly more air to make rated power. But the cowl was not redesigned at the time for whatever reason. I don't know if it was the total air volume or the S-turn that had the most impact, but the combination of those things produced a sub-optimal induction system, which was mitigated with the ram air system, which does produce a small power increase when opened. This small but observable power increase means that with ram air closed, the induction system is definitely not "more than adequate for the needs of the engine". Ergo, those airplanes can get a small performance benefit from a more free-flowing filter, at the expense of allowing more foreign material entering the engine. Mooney dropped the ram air in later models with better cowls and induction designs because it provided no benefit, and I speculate that in those designs, the airflow provided by the stock filter is indeed more than adequate for the needs of the engine as you say. If that's true, there would also be no benefit to a more free-flowing filter (and the dirt it allows) in those airplanes, because the extra air volume such a filter would allow wouldn't actually benefit the engine. That doesn't stop 201 and Ovation owners from considering "better" filters in search of a few extra horsepower, and I'm not smart enough to say they're wrong to do so.

Intuition is not data, and is frequently wrong. If it were a simple matter of "more area = more flow", then the best solution would be to speed tape 2 or 3 or 4 filters in front of each other at the airbox. That's even more media area, so it must mean more flow, right? Also, if I remove the air filter altogether, the filter media area is zero, so does that mean the flow rate is worse with no filter than with a filter? Obviously this doesn't pass the smell test. Probably because the flow equation in a real world installation is more complicated than you're making out. If we want to go full nerd, the relevant equation is Darcy's Law, which models the flow of fluid through a filter as a function not only of filter area, but also of pressure differential across the filter. I'm not smart enough to describe the relationship between pressure drop and number of pleats in an aviation air filter, but I'm willing to bet there's a dependency. Anyway... I promise I'm not just trying to be a contrarian about it. But saying "it's intuitive", or "Science" isn't going to sway me. Data or nothing, and the only data I'm aware of says that for products actually on the market, "more flow = more dirt". Again, and I can't emphasize this enough, that doesn't mean "more flow = less engine life". There is some quantity of dirt below which there is no meaningful impact on engine life, and high flow filters may well be above that threshold for most operators.

Sounds like the claims of gasoline additives that improve mileage. Maybe a grain of theoretical truth, but never seems to hold up when put to a practical test. I'm only interested in practical data, and I'll believe it when I see it. If you've actually got some, I'm all ears. The most recent serious, scientific look I've seen on this stuff is from a Kitplanes article: https://www.kitplanes.com/oil-filter-testing/. The topic is oil filtration rather than air, but the principles are the same: The report can be summarized as: "more flow = more dirt". Again, though, one can't necessarily jump to the conclusion that "more dirt = less engine life". For any given airplane in any given environment and working life, there is some level of filtration that's adequate, and anything better than that isn't meaningful. What I do think is meaningful is that if you're really going to go chase this idea of better intake flow, the first thing you should do is fly a short flight with no air filter at all and gather data. You're not going to destroy your engine doing so, and it will show you the limit of what's possible. In those vintages of Mooney that have a ram air door, you can perform the experiment simply by opening the ram air door, perhaps shortly after takeoff when you're away from most of the dirt.

Chiming in with the "no free lunch" reminder. Nothing wrong with chasing a few extra horsepower, but any filter that allows more air to flow will by definition allow more foreign material into the engine as well. There is no magic filter material that overcomes this universal truth. That said, my understanding is that essentially any filter is "adequate", and there's of course no simple calculation for how many more days/weeks/months you can run your engine with better filtering/less airflow, vs. an alternative. The best reason to use something other than a Brackett filter is probably just to avoid the oily goo that comes with it, performance and longevity questions aside. I think about this every time I change our filter and wipe the goo off my hands and the airframe. But I've yet to try an alternative. Most recent thread I can find on this recurring topic:

I appreciate the mention, but I'm unavailable for this sort of work at the end of July due to other flight instruction commitments. Any of the other options discussed above are fine choices.

It's my understanding this is no long true, with the release of the AV-HSI converter unit. See https://aviationconsumer.com/avionics/uavionix-av-30-efis-glideslope-arinc-ap/ This is part of the teething pains I talked about. I think uAvionix felt like they had to get "something" out on the market ASAP a few years back to stay relevant in the face of competition from Garmin and Aspen. But the original AV-30 implementation lacked features, and had issues like excessive DG drift. I feel like uAvionics essentially sold beta units to customers, then fixed the issues and missing features later, but I guess I don't begrudge them that strategy too much - it's used a lot in the tech industry. Anyway, that's all history now. Seems like most of that stuff has been addressed, and is arguably not relevant to potential new customers.

The physical size of the display in the AV-30 is even smaller than that of the GI-275 due to the bezel knobs and buttons, and it has a lower resolution. That makes it a non-starter for me in an IFR aircraft, but again, this has more to do with the quality of my eyeballs than anything else. The AV-30 backup battery is advertised at 2 hours typical. That's considerably less than a G5, but comparable to a GI-275, and most people agree that 2 hours is plenty of backup time. My opinion of the AV-30 is negatively skewed by the teething problems uAvionix had bringing the product to market, but those seem to now be solved, and as of 2025 lots of people seem to be happy with it. Do note that if you eventually install a second AV-30 as a DG/HSI, it's critical to pay extra for the Av-mag external magnetometer even though it's "optional". Without the magentometer, precession of the instrument is worse than a mechanical DG, arguably unusable in IMC. Overall, I'd be perfectly happy to fly behind an AV-30 in an aircraft intended primarily for VFR. But I wouldn't put one in an airplane I really intended to fly in the clouds, except as a backup to something else. Just my $0.02 of course.

I have specific experience with this. We did exactly what you're proposing, about 4 years ago: installed a single G5 configured as an ADI to replace our failing vacuum AI. We retained our vacuum system to drive the vacuum DG and the Brittain autopilot. The ADI installation is relatively simple compared to an HSI installation, and any A&P can likely do it competently. All you need is the G5 itself, a connector kit, and pneumatic couplings for the pitot/static lines (collectively the cost of all this is a bit more than the G5 unit by itself, but not much). No integration with nav radios is required, though you do have to either connect to the RS-232 GPS output from a GPS, or connect an external antenna to the GPS receiver in the G5 itself. Any competent shop will have done a bunch of G5 installations, and should be able to give you reliable quotes for various options with reasonable labor charges. Make sure any quotes you get include the connector kits, so as not to be surprised by a small-but-non-zero additional charges over what you might see vs. online advertised prices for the G5 unit alone. One thing to be aware of is that if you have a standard 6-pack panel with the DG directly below the AI, and you do not pay extra for the equipment and labor to flush mount the G5 ADI, the outer bezel of the G5 unit may impinge on your view of the very top of the DG. The very top of the DG is of course the most important part, so that's a concern. I didn't think this would be a big deal, but it did moderately annoy me for a year or so until we bit the bullet and installed a second G5 as an HSI. Whether you'll have this problem depends on the particulars of your panel, seat position, and height. You should make yourself a little mock-up of the outer bezel of the G5 to see how things would look in your airplane. I disagree that a GI-275 is a no-brainer preference over the G5, though it's a complicated decision. I've flown both, and yes, for about $2K more the GI-275 has more features, a higher resolution display, and it won't have the non-flush-mount problem I discuss above with the G5. It's a wonderful instrument. But it's also a physically smaller instrument, despite its higher display resolution. So if you really take advantage of all the extra EFIS features (synthetic vision, airspeed and altitude tapes, VSI indications, GPS track, flight path marker, slip/skid indicator), you wind up with a very busy display on a very small screen. This would have been fine with me in my 20s and 30s when my eyes were great. Now that I require optical assistance, I prefer simpler and larger displays, and to some degree a GI-275 ADI feels more like a win in the bragging rights department than it does in actual utility. The display can be simplified by disabling features, but then you of course wind up asking yourself why you paid more for them in the first place. So... it's a big cost/benefit personal preference issue, and you shouldn't choose something on the basis of pictures on the internet or what others argue here. Make the effort to go see some actual installations in actual airplanes, ideally in a Mooney similar to yours. Finally, consider how likely you are to install a second electronic gizmo when your vacuum DG/HSI eventually fails. HSI installations are complicated by interfaces to magnetometers, radios, and autopilots, and the GI-275 has some advantages in that department due to native interfaces it supports, which require an extra converter box (GAD29) in a G5 installation. Those advantages mitigate some of the additional cost of the GI-275 unit itself vs. a G5. Hard to say how much without a specific quote, so you might want to get quotes on dual ADI/HSI installations even if you intend to retain your vacuum-powered DG/HSI for now, just to understand the future cost implications.

Ironically, the only place I've been faced with it is here in that same Denver area you speak of, during that same small window of flyable IMC in late spring (it was really good this year, I've flown more actual IMC in the past month than in the last several years). In my experience the last decade or so, flyable IMC here has been either totally uninteresting, or LIFR. Between students and my own practice, I've only flown a few dozen approaches in IMC past the FAF. But I have several stories of not seeing approach lights until right at minimums, and needing to go missed for real. This has led me to something of a contrarian speech for my students, about personal minimums vs. what is often taught in training material. The training material suggests that one might step down their personal IMC minimums over time. Start with ceilings 1000' AGL, step down a couple hundred feet at a time as you gain experience. That strategy just can't be executed in the Denver area, there aren't enough opportunities. If you want to fly actual, you need to be proficient enough to go fly when airports are reporting LIFR. I'm willing to do that partly because I've flown all the local approaches with ATC hundreds of times, and partly because there is essentially always good VMC within range of the airplane (I've yet to have to actually leverage that, but nice to know it's there). So I think this just gets back to the fact it's situational. I'm not some steely-eyed missile man because I'll fly low IMC in the Denver area when it happens. It's a very specific environment and set of skills I'm comfortable with, after a lot of practice. I'd bet some people here are very comfortable with coastal fog in a way I'm not, others with east coast weather, etc.

Agree with @MikeOH that it's less about the current weather and forecast at my specific destination than the overall weather scenario. I wouldn't bother to drive to the airport if it's 200 and 1/2 for hundreds of square miles in any direction around the airport I'm trying to get to. But if there is nearby widespread good VFR and forecast to stay that way, my minimums for planning the flight are basically zero/zero, and my minimums for commencing an approach are anything reasonably close to the legal minimums for the approach. Some flights don't fall into these neat categories, of course, particularly if I've never been to the destination before. Then I agonize like everyone else, and do my best to err on the side of safety.

Hard to say at low resolution, but looks to me like there is significant brown discoloration at the break point on the shaft. A clean break on an fully intact part would be much shinier, no?

Oh c'mon, this is just stupid. The fact I only pointed out 3 examples doesn't mean there have only been 3 failures ever. Look at the picture yourself, from that thread. More than half of the threaded portion of the attach point for the plastic wheel-shaped knob is broken off. This is not just a failure of the plastic. If you want to quibble that a different part of the metal shaft broke vs. the OP in this thread, then sure, it's a different failure mode. Whatever. Same as the OP in this thread, the shaft broke close to the base such that it could have only been actuated with tools. No, I wasn't the guy with my hand on the switch, but I believe my airplane partner when he said he applied only a moderate amount of force. Some of you guys seem to think these switches can only break when a neandrathal lunk applies hundreds of pounds of force to the switch without pulling it out of the unlock position. I'm not buying it. If that's what's really happening, why do the little locking tabs that prevent the shaft from moving - which are smaller than the shaft itself - show no damage in the photos above? I'm sure that's true when they're brand new. As I've already explained, I think decades of normal use - with a few incorrect operations exactly as you describe from time to time - can fatigue the metal. As I recall, you're the same guy who keeps arguing that properly lubricated landing actuator gears will also never wear out. You have a lot more faith in the lifetime of this stuff than I do, but you do you. Good point. Why do some airplanes have big, robust gear switches, if there's nothing wrong with the design of the tiny little ones? Now we have 4 stories. But I guess everyone who broke the gear switch - the OP's mechanic and my airplane partner and JTR's wife included - are all ham-fisted idiots who deserve what they get. Roger that.