AI Back-Ups
Your recent article on the various gyro back-up options was interesting, especially in light of recent accidents that were the apparent result of loss of vacuum. I agree that a second electric AI is probably the best bet, especially for single-engine aircraft. I think that the following points are sometimes overlooked when considering backup options for the vacuum pump or AI:
The vacuum pump is guaranteed to fail eventually, and should therefore be replaced periodically before it does. Perhaps every 500 hours.
If youre going to install a second engine-driven vacuum pump, then dispense with the electric clutch arrangement and let the pump run full time, just like on a twin. Plumb both pumps through check valves so they share the load, thus increasing the life of each pump.
If you install a new (zero-time) second pump now, you will statistically eliminate the chance of both failing at the same time. Include a second vacuum gauge for the second pump and check it often.
I think that a lot of standby pumps fail just when you need them because of the shock effect of a sudden start up to full RPM and full load , rather than the more gradual start up RPM and load on the primary pump that occurs when you first start the engine.
AIs and other gyros do not necessarily fail completely. Usually, the gyro just slows down and becomes lazy or slow to respond to changes in attitude. If you only have two gyros and you suspect one has failed, there’s no sure way of knowing which one. A third gyro provides a best two-out-of three method to quickly determine which gyro is bad.
-Peter McLean
Tega Cay, South Carolina
We agree with the idea of running a standby vacuum pump full time, given the troubles so many owners have had with the clutch arrangements and the standby pump tanking when its most needed.
Were lukewarm on the idea of proactive vacuum pump replacement, however. In our experience, infant mortality for new and rebuilt pumps is high. If a pump has survived 500 hours, it might just survive another 500. In our view, carrying a spare pump in the baggage compartment makes more sense, plus a standby AI to get you down safely.
Finally, three gyros seems like overkill to us. Two AIs-one primary and one standby-plus a turn coordinator strikes us a reasonable balance of redundancy against cost.
Enjoyed the article about gyro back-ups. The concept of single-point failure is important. While we were looking at back-up options for our airplane, my next-door neighbor told me a harrowing story.
He had to take control of his Learjet during a low-IMC departure because the co-pilot flying was following his (failing) AI and flying in an increasing bank that was getting them close to terrain. The co-pilot was experienced but didnt quite recognize the subtle failure mode. The second AI saved the day.
We decided to go with an electric AI and have been happy with it. Youre right that theyre a little more sluggish than the primary, but we fly some approaches and maneuvers with it every year and its more than adequate. There have been no maintenance problems with it over eight years.
Its worth emphasizing the importance of placement close to the other critical instruments as shown in your pictures. We know of one airplane where the back-up AI is on the far side of the cockpit and it is very difficult to effectively scan over there.
-Andrew Doorey
Wilmington, Delaware
Dial Eastern States Kudos
In late 1993, I decided to bite the bullet and have my 1976 Skylane repainted. The original Cessna paint looked…well, original.
After reading an article in The Aviation Consumer on recommended paint shops, I spoke with all of them within 500 miles or so of where I lived in New Jersey and visited two of them, plus one shop which wasnt on the recommended list to see examples of their work.
I finally decided on Dial Eastern States Aircraft Painting, Inc., in Cadiz, Ohio. I reserved a time slot and much to my amazement, when the time came, they were ready for my airplane. The work was timed to coincide with the annual inspection, which they were able to perform, as well.
Dick Guenther, the owner of Dial Eastern States, spent at least an hour with me going over various photos of airplanes he had painted in order to select a design.
He made several suggestions as to how they could be improved. The paint job which DESAPI (their acronym, not mine) did was superb and even six years later, it draws rave comments. Just two days ago, a controller asked me whether my airplane was one of the newer Cessnas.
The work included several items which I had not requested, such as outlining the N-numbers in a contrasting color. Dick promised that the paint job and annual would be completed in three to four weeks. I made a reservation to fly commercially to pick up the airplane with the four-week time frame in mind.
Two weeks after I left the airplane with DESAPI, I received a call that it would be ready at the end of the third week and I had to change my flight reservation.
The paint job looked great, the annual inspection was done we’ll and I have had no complaints whatsoever with anything Dial Eastern States did. Oh yes, their price was on the low end (but not the lowest) of all the shops I spoke with.
One more thing: Dick and his wife drove me to and from the Pittsburgh airport to catch my commercial flights. No charge.
Would I use DESAPI again? You bet!
-Peter Sachs
via e-mail
GPS Shock
Your February 2000 issue arrived with good timing. I just purchased a factory reconditioned Bendix/King KLN 90B via Avwebs online auction for what I think is a good price: $2845, plus $15 shipping.
When I read Larry Anglisanos GPS on the Cheap article and saw his estimate of $2000 for an IFR install, I was feeling pretty good about the choice Id made to make my 1984 172RG a /G for approaches. Imagine my shock, then, when I got the installation quote from my local shop, Southern Avionics of Mobile, Alabama:
Materials:
1 KN-72 VOR/LOC converter: $1220
1 GI-106 (Garmin) indicator: $1370
1 MD-41-324 annunciation unit: $925
Miscellaneous hardware: $100
Cooling fan: $170
Subtotal: $3785
Labor: $1760
Total: $5545
The shop says some of this extra hardware is part of the curse of my existing ARC avionics and any GPS installation would require the same stuff. This quote, by the way, was part of an overall quote including an S-TEC 50 autopilot and a PS 1000 four-place intercom.
I went back to Southern Avionics and met with the owner, Tom Greer. I told him his estimate was killing my budget. After some discussion, he said that moving the ADF indicator over to the co-pilots side and using the hole for a third CDI dedicated to the GPS would eliminate the need for the KN-72 VOR/LOC converter and associated labor. Also, a less expensive Garmin GI-102 indicator (no glideslope) could be used. Total savings: $2000.
The total approach-certified, installed cost for the KLN 90B is about $6400. This is enough of a savings versus the installed cost of any new unit that I am going ahead. One more lesson learned: Beware of going to the avionics guy and telling him exactly what you want done. Better to discuss what you need, what you can do without and whats most important to you. Give him room to get creative and perhaps save you some money.
-Dan Luke
Daphne, Alabama
Vertical Compasses
With reference to your article on vertical card compasses in the December 1999 issue of The Aviation Consumer, I spent years trying to get the Precision Vertical card compass to work right in our Glasair.
We hung it overhead, but I wasnt about to use the double-stick foam thingie they offered. (It would never hold up in the Phoenix summer, looks awful and put the compass too low for us.)
At first, I used some hardware store rubber mounts, but these transmitted too much vibration to the unit and it would randomly point north in flight. If I loosened the screws just right, it would mostly work, but go nuts in turns.
To make a long story short, I did two things which have made it work very well: 1.) I put a little bit of lead (tape-a-weight) in the housing to dampen vibration. (more inertia) and 2.) I hung the unit with Velcro. (I used VHB Velcro for the Phoenix heat.)
The Velcro isolates the vibration enough so that the unit works we’ll in flight. As a bonus, in a crash (or just getting in and out of the plane sometimes) the Velcro breaks free on impact. I attached a safety string to catch it when we knock it with our heads. Isnt homebuilding neat? Didnt even need an FAA approved string.
-Mike Palmer
via e-mail
