The International Cessna^® 170 Association

Over the last couple of months I've noticed my airspeed increasing a few knots at the same power setting. Borrowed a tach checker and found my tach reading about 600 RPM low. It is the original from 1963 according to the logbooks and I plan on hanging onto the bird for awhile. Any suggestions for a digital tach and what do you think of them? Any inputs appreciated, thanks. It is a 1963 C172 with O300C engine.
Jim Rogers

Digital instruments are excellent for exacting readouts. They have great attributes for diagnosis, such as a mechanic's digital tachometer which you likely used to confirm your original tach's accuracy.

They are less useful in daily aircraft operations, in my opinion, because they do not demonstrate "trends" well. (They require mental math to gauge trends.) An example is an airspeed indicator or altimeter. Although one can readily determine that the digital numbers are increasing or decreasing, that information does not relate trend info as readily as analog instrumentaton. Seeing 68, 67, 65 in an instrument does not translate as readily the hazard of approaching stall speed as an analog meter because the relative speed of the changing numbers don't equate to the speed at which the needle is approaching the lower limits of the indicator. (Humans are not digital...we're analog. When EFIS (glass cockpits) were developed, it was quickly discovered that the video screens needed to resemble ordinary analog instruments before the pilots could rely on their information quickly. Now we have cathode-ray (TV) tubes that try to look like analog meter movements, and "trend" displays that electronically mimic analog meter movements, because, when time is of the essence, digital readouts can't be quickly assimilated into useful information.)

Digital EGT gauges are something I have experience with. I found them to be distracting as in cruise flight they constantly flickered 1435, 1438, 1437, 1434, 1436, 1435, ... and on and on ... instead of a relative steady needle sitting at the 1435 point. In other words, no "damping" of the digital gauge is possible, such as occurs normally in an analog meter. And besides, what possible use of such micrometer measurement could be made? It's like the proverbial excersize of measuring something with a micrometer, marking it with a grease pencil, and cutting it with a hatchet.

Then there's the matter of nighttime flying. The digital gauges are usually lighted internally. That one bright gauge in the panel was a distraction. If it were a backlit LED display, in order for it to be read reliably at night it had to have a different brilliance-level than the gauges surrounding it, and it was subject to freezing in the winter and overheating to complete blackness in summer. (LED's have improved a bit in the latter regard in recent years, but they are still somewhat subject to temperature failure modes.) If it were a gas-plasma display, it ruined night-vision, and was an expensive failure when it burned out. And in the case of a tachometer, it would not relate to green arcs, yellow arcs, red-lines unless the guage also had different colored lights. While lots of different colored lights keep children amused and occupied, I felt it made the cockpit look incongruous and disorganized.

Then there's the electrical failure modes. An electrical failure resulted in instrument failure. A mechanical or self-exciting gauge did not suffer in that regard. Mechanical tachs indicate whenever the engine turns regardless of generated electricity.

The tachometer might be more useful as a digital meter than some other gauges perhaps, but I'd prefer a standard analog meter movement, not only because it looks more original, but also because it is less complicated to install, less expensive, and requires no additional certification.

If this wasn't what you wanted, it's your fault. This is what you get when you ask my opinion.

gahorn wrote:If this wasn't what you wanted, it's your fault. This is what you get when you ask my opinion.

EXACTLY what I was looking for. It's a good bird and I plan on keeping it a long time. I've got about 10 thousand hours sitting behing EFIS gauges and I prefer steam gauges in my own bird. I know what you mean by trends. That has been one of my concerns.
Thank you. Jim Rogers

Jim-
I have a Horizon P-1000 digital tach in my C-170. Although you can see in the picture my panel is a mix of steam guages and digital I don't have any trouble interpreting any of them. The last two digits fluxuate but you are only looking at the first two anyway. It's interesting to see how turbulence and small pitch changes affect the RPM. I upgraded mostly because all the original instruments were just that, original and worn out.

The P-1000 had to be field approved in the 170 with an O-300-D, but if I recall correctly, is STC'd in all C-172s.

Not only do you eliminate the troublesome tach cable and its firewall penetration (the P-1000 ties into your mag switch) but the P-1000 tells you if one mag or the other has trouble or is not working (like when you don't turn the mag switch all the way back to both during run-up). It has a digital hour meter that Horizon will initialize at any hour reading you choose as well as a maximum RPM trap for each flight. It also has annunciator lights to indicate if you are in the green, yellow, or red RPM ranges. Notice the yellow light shining in the photo at 2460rpm.

I am sold on the P-1000 but the EI digital tach works just as well if you have an extra 2-1/4" hole and need another 3-1/8" hole in the panel. I just don't like the chintzy looking lights on the EI. Click on the picture to enlarge it. P.S. George- those are still my shoes with the red paint on them, not Ina's. My nephew (An A-10 fighter pilot with a tour in Korea, two tours in Afganistan, and the current U.S. Air Force A-10 Airshow Demonstration pilot) is in the left seat.

Richard I'd a thought you'd have changed shoes since the last time we saw that picture.

I've flown from the right seat in 170s that had a digital tack and not being the pilot at the controls had nothing better to do than watch that last digit bounce up and down. It sure was distracting. I can't tell you if I would even notice if I'd been at the controls but the distraction was enough to through of my navigation skills as Jim Wildharber and I made a futile bombing run at the Galveston Convention.

As a pilot that flies a mixed analog/digital cockpit at work I can say that it is much quicker to glance at an analog gauge and know whats going on with the aircraft but when I want to know precisely what percent N1 the bleed valve is opening or closing you can't beat the digital gauge.

The perfect world would be an old fashion analog tach for quick reference and a digital readout for those long periods of cruise flight when you have nothing better to do than fiddle with your vernier throttle and mixture to set the perfect RPM and mixture using your six position digital CHT/EGT for the best fuel flow with your digital fuel flow meter. Just hope your not on a bombing run when doing so.

I still think they look like "girlie" shoes!

George
I have seen the shoes, and unless Ina moonlights as a clown they aren't hers!

She FLYS WITH a clown! (and he has the steadiest digital gauges I've ever seen! LOOK AT THEM! THEY DON'T FLICKER AT ALL!

What stands out for me is that Richard appears to have the shoes on the wrong feet!!!!!!

That flight has been occuring now for several months... perhaps he has a bladder-pressure-light and has his legs crossed! (it's a girlie thing!)

I tried to reply on this subject Sunday on my daughters computer and had it about ready to send and then lost it so I am just now piping in I think Richard has pretty well said what I was going to say, however as one who just went to the Horizon digital tach, I will say that at first the rpm's changing was a distraction and I couldn't believe the engine could vary that much, but YES IT DOES Get in a cruise in smooth air and it will smooth out. Also its neat to set the rpm & be able to tweek it to exactly what you want it to be. I went to the digital because I got tired of replacing tach's, tach cables and the tack drive housing. The Horizon is a bit expensive, but I am of the opinion that it is worth it (who ever thinks after spending $$ on something for their airplane that it wasn't worth it ), but one positive thing is it is accurate and most mechanical ones aren't. It doesn't do much for originality, but I have yet to see a 170 that had not been changed for one reason or another from the original (transponder, tube radio's, etc). Installation is straight forward. I don't remember the brand, but I seem to remember seeing some new style instruments in one of the catalogues that would give you both an analog reading and digital in the same unit. It may not have been a tach, but may have been oil temp, oil pressure, etc.

Frank we're going to have to reduce your judging score next time we judge your C model. Darn shame to because it almost won Best orignal C model at Branson.

N9149A wrote:Richard I'd a thought you'd have changed shoes since the last time we saw that picture.

Bruce- I wore them today! Those are my favorite shoes and I would have bought a new pair by now but it appears Rebok doesn't make that style any longer so I'm going to ride 'em into the ground!

gahorn wrote:I still think they look like "girlie" shoes!

George- I don't know what kind of girls you're used to running around with but if these size 12's are "girlie shoes" to you, I sure don't want to meet them!

johneeb wrote:What stands out for me is that Richard appears to have the shoes on the wrong feet!!!!!!

There aren't many pilots I feel comfortable enough with to totally relax, adjust the seat all the way back, cross my legs, just sit back, and enjoy the ride, but it's nice when you run into one.

170C wrote:I think Richard has pretty well said what I was going to say..... I went to the digital because I got tired of replacing tach's, tach cables and the tack drive housing...... one positive thing is it is accurate and most mechanical ones aren't.

Thank you Frank! That was the whole point of the post.

hilltop170 wrote:...George- I don't know what kind of girls you're used to running around with but if these size 12's are "girlie shoes" to you, I sure don't want to meet them!

IMG_7421_1_1.JPG

Well..when you blow them up to actual SIZE... I can see what you mean! (thank God the internet doesn't transmit the smell!