Turn Coordinator Gyro
 

Hey,
I have what might be a dumb question, but I'm OK with asking it. :cool:

I went to fly a coworkers airplane today, a 1979 Cessna 172N (I have flown it many times), and when I flipped the master switch as part of my preflight inspection (fuel guages, nav/beacon lights, flaps, etc), I heard complete silence. The electric gyro in the turn coorindator never spun up. I never noticed if the red flag indicating a failure/power was on or not.

If I remember right, this is not a required piece of equipment for a daytime VFR flight. I let my mind wonder, and started thinking; was this an electrical problem in the turn coordinator, or was the problem further upstream? I didn't want to take off anyway and end up with an electrical system failure/fire due to a short somewhere.

My question is, did I let my mind get the best of me, or did I make a smart decision?

Did you check to see what the power source for that instrument was? Not all are electric. Did you check to see if a circuit breaker was out?

I can't speak for UK regulation, but generally speaking, if it's installed, it's got to work. If it doesn't work, it needs to be deactivated and placarded inoperative, so long as it's not required for the kind of operation in use (daytime, VFR, for example).

Some gyros can be remarkably quiet. A quick taxi check and a few turns on the ground would have indicated whether working.

It is definitely electrically powered, and I didn't see any breakers that were tripped. And I've flown this plane before, and it is definitely easy to hear (at least before...)

This plane's turn coordinator is definitely electrically powered. I've flown it many times, and its noise is definitely noticeable.

If a Cessna 172N has a factory original turn co-ordinator, it's electric. If it has a little red flag, it's electric. Those Turn co-ordinators are notoriously unreliable. They are DC electric, which means they have brushes which run on a commutator. The brushes wear, and the electricity stops flowing. Add to that, a commonly huge mess of greasy carbon inside. The red flag is not a 100% indicator of this failure. Listening is better, until you're in flight.

I have, and recommend to anyone who's going to keep the plane, to have the maintenance shop carefully remove it, and discard it with predjudice. Then, go and buy an AC version (which could run $1000) of the same instrument. You pay in the beginning, but it will very likely outlast the plane, with zero maintenance ever, and 100% reliability forever.

Or, if you're thinking auto pilot, now's the time, as some come with a replacement turn co-ordinator.

Though tempting, it would actually not be permitted to replace the electric turn co-ordinator with the older and more reliable vacuum turn and bank. This is because the of the design requirement for that aircraft that the turn indicator not be powered from the same power source as the attitude indicator - just in case one quits. Oh yeah, you know about that now! The vacuum pumps are a little more reliable that the factory original turn co-ordinators, but not much!

It's always a smart decision to consider the unknown causes of electrical problems, and not fly, if in doubt. My experience would have allowed me to continue the flight, as long as everything else was working properly. If one of those quits, my experience is 99.9% instrument (totally benign to the rest of the aircraft), 0.1% upstream problem (and that will be detectable by observing other things wrong/breaker popped etc.), probably still benign to the aircraft, but worthy of consideration.

Just noticed you're from Texas, ajstoner. That being the case, your original question seemed to pertain to whether or not the turn coordinator was a required item, as well as whether or not the turn coordinator had a problem.

Without actually being able to see the airplane in question, it's impossible to diagnose your turn coordinator problem. It could be the instrument, a circuit breaker, a loose cannon plug, or perhaps an additional power switch for the turn coordinator on the panel (some aircraft use them to prevent the turn coordinator from running when it's not needed).

The day VFR instrumentation and equipment requirement to which you referred is found in 91.205:

§ 91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.

(a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.

While the turn coordinator isn't required for day VFR, it's still got to work if it's installed, or be properly altered. Like most reguations, reference to one reg usually won't answer your question. The answer, then, is found in 91.213, which deals with inoperative equipment and instruments.

The point of that paragraph is that everything must work. Three types of relief are found. One is the MEL, if you have a minimum equipment list for your aircraft. It's got to be specifically designated for your airplane by registration and serial number, and approved by the FAA.

The second choice is removal or deactivation. This is the more common method in light, private airplanes. The third option is to obtain a special flight permit, sometimes called a "ferry permit."

Note that to get the aircraft is in a "properly altered condition," one also needs to visit Part 43 to understand the maintenance and paperwork requirements...Part 91 only tells part of the story.

It's probably the brushes have gone, send it off for repair.

That may be, but I'd second the suggestion by Pilot DAR. If one is going to spend the money to overhaul anyway, go with the upgrade and don't worry about it again.

Conversely, for one who might decide to operate as is, a simple short can turn into a fire or cause other problems.

I worked for a corporate flight department some years ago as a copilot and director of maintenance. In addition to the corporate aircraft, several of the board of directors shared a Cherokee, which was also my responsibility. The TC wasn't working. I discovered that it had a wiring issue, not the least of which was that the wiring bundles were binding on the controls. While a fire was a possibility at the time, it wasn't nearly a concern that the binding controls were. I ended up re-routing wiring, moving instruments, and tying everything off. It came out well...but the point is that a small problem may be indicative of something unseen.

If the instrument is going to be replaced or repaired, consider going with a better instrument.

Dick Collins, formerly of Flying Magazine, championed replacing turn coordinators and turn and bank indicators with a second attitude gyro. This makes a lot of sense and provides a lot more useful information.

Umm... I must be missing something here.... A second attitude gyro in place of a turn co-ordinator, would leave you certain about the aircraft attitude, though not informed about the turn, or slip of the aircraft at all! The TC and attitude indicators do two rather different jobs.

I'd stick with the configuration which Cessna offered from the factory, when it comes to primary flight instruments....

Thanks for all the information! I was always hesitant about the inop equipment (if it wasn't required for that type of flight) just because of that deactivation/removal part.

Is the turn coordinator something a pilot can deactivate without alot of effort? I don't remember seeing a breaker called out for it.

Well, that's a bit tricky, and subject to some interpretation.

In Canada, the closest I can find to this would be in the list of tasks a pilot may carry out (does not require a maintenance person):

(27) deactivating or securing inoperative systems in accordance with sections 605.09 or 605.10 of the CARs, including the installation of devices specifically intended for system deactivation, where the work does not involve disassembly, the installation of parts, or testing other than operational checks;

Cessnas of this type did not leave the factory with circuit breakers you could pull. The earlier Cessnas had fuses, but often the circuits were joined, so that you'd loose several systems by removing one fuse.

So, personally, I would be comfortable removing the cannon plug from the back of the turn co-ordinator, TYING IT PROPERLY OUT OF THE WAY, AND CONFIRMING THAT NOTHING IS FOULING BACK THERE! then marking the face of the instrument "inoperative" and flying within the suitably appropriate day VFR limits for the aircraft and pilot skill.

That said, that's me. You have to do only what YOU are comfortable with, and within FAA regs, not Canada's. It is possible that you could be asked to justify what you had done - satisfy yourself you could!

Pilot DAR

Richard Collins did NOT advocate removing the turn and slip, what he did advocate was replacing the traditional instrument with a second AI AND installing a 2 inch turn and bank somewhere else in the panel. Without a turn and slip the aircraft would NOT be IMC legal (IFR in IMC), but might be legal under VFR (see posts above).

You are therefore correct, you need at least a means of keeping the ball in the pot.

With glass sytems the ball is often combined with the backup AI which works just fine.

Richard Collins was however correct. Relying on one AI (particularly if vac powered) is daft - however good our instrument flying skills, it is no fun completing the mission with just a turn and slip.

I dont think it does make sense.

The information you get from an AI doesnt change. The purpose of having a second is to take over from the first, when the first fails. The only extra useful information is the ability to check one against the other BUT you still need to be sure you know which one to believe so there is an additional inherent danger in having two of which you need be aware.

Regarding the above, is it sufficient, according to the regs, to simply tape a piece of paper with the text "U/S" (or something to that effect) over an INOP instrument? Or do you really have to physically remove the instrument from the aircraft, or at the very least disconnect it from power/vacuum/pitot/static sources? (The latter requiring a trip behind the panel of course.)

I have, on a number of occasions, taken off with an U/S instrument that was not required for that type of flight (day VFR), and simply taped a piece of paper over said instrument - more to prevent confusion than anything else. Was I legal? (In an JAR member state, but I would be interested in the FAA position as well.)

And as an aside, if you disconnect a pitot/static instrument from the pitot/static tubes, do these connectors auto-seal or do you have to put some kind of plug in them, to prevent false readings on other pitot/static instruments?

The second AI would not meet the intent of the design requirement 23.1321:

The instrument that most effectively indicates the attitude must be on the panel in the top centre position; [/font]

in the position of the TC.

Whoa there! The discussion of the pilot disabling the turn co-ordinator is definately limited to that instrument only. Any maintenance on any pitot static or vaccum instrument very certainly requires properly trained personel. In particular, you should expect that airspeed indicators and altimeters are required to be servicable for any flight.

That said, an instrument which is legally not required, can be temporarily labled "inoperative" in place if secured properly. This is with the expectation that it will be removed by maintenance personel in the near future, either permanently, or for repair. The only reasonable exception I have seen to this is where a secondary instrument like an ammeter is in a cluster (can't be singularly removed) and it has been disconnected, and replaced with a new, separate ammeter. Then you'll sometime see a permanent "inoperative" on its face.

The replies on this thread are not intended to confer maintenance authority to pilots. What you have here, is guidance telling you what, and where to look for the regulatory information you need to properly care for the aircraft you are flying. Know your privilages, responsibilities, and capabilities! Saying that you read it on PPRuNe is going to look pretty poor, when challenged by your local authority for unauthorized maintenace of an aircraft!

Pilot DAR - you have lost me - the second AI is replacing the T and S not the primary AI.

I understand that the proposal was to replace the t&B/TC with a second AI, in that position, but doing so would not conform to the design requirement stated. Though you still would have the original AI in its proper position, the design requirement does not provide for also providing that information in a non-conforming location on the instrument panel (and prodiving the expected T&B/TC information elsewhere).

In instrument flight, there is an expectation of an effective instrument scan, with the pilot finding information in the panel location expected. A second AI in a non conforming location has to potential to disrupt this. Not having the T&B/TC there certainly does, were "needle, ball and airspeed" flying become necessary.

If a second AI is proposed, it should either be in the corresponding copilot's panel location, or top centered, as is common for glass cockpit aircraft.

Instrument location in the panel is one of the times when standardization for flight safety is a very good thing.

No, I dont think so.

Compare cockpit photos of a DA42, Cirrus and Mooney - each has a different location for the backup AI. The reversionary AI on the G1000 and R9 throws up yet another placement.

I do accept that some aircraft might have a standby AI in a varied location, as the panel space and other considerations permit. Presuming that such placement is defined in the original approved configuration of the panel, there will have been an assessment of the design, and following that, an approval. If the designdid not follow the design standards, there will have been consideration of the affect, perhaps even an equivilent safety finding. That information is not generally available. Ideal? Perhaps not, but the best that was possible with the design. Sometimes things like how much depth behind the panel is available dictate where certain instruments end up.

If the AI was placed "somewhere" on the panel as a part of a retrofit, or modification, either it went through a similar evaluation and approval process, with a DER/DAR, or local equivilent, or perhaps it is just not approved!

Glass cockpits are certainly causing a need to rethink panels in general. I'm sure that by the time I retire from flying, round instruments will be as uncommon as tailwheels are now!

By the way, the first time I flew G1000 was in a Caravan. Neither of us had flown the G1000 before. I flew while the other pilot worked out how it worked. The only thing that neither of us could figure out was where the slip indicator was in the display. Only after some deliberate wild skids and slips to see what moved, did we work it out. A good idea ultimately, but a little too subtle!

Yes, me to, I thought the same.

The position of a stand by instrument is a compromise. There will be a turn and slip somewhere on any IFR certified panel. The designers obviously believed the location of the turn and slip was adequate for partial panel. That is the first hurdle. The second hurdle is are you better with an AI in the same position as a the turn and slip, and if you are, why didnt the designer put one there? Well he didnt put one there because of cost. They either both need to be electric on different systems or electric and vac. Either way it is not an insignificant cost.

Combine both AI with slip indicators or even just a slip indicator on the main AI and you have a fine solution. Alternatively put the slip indicator somewhere else because it is hardly essential if one or other AI is working.

However if the turn and slip is certifiable where located I dont see what difference there is if you put an AI in the same hole instead of the T and S? It essentailly does the same job - just better.

A lot of people install a 2nd horizon in place of the T&S/TC because it is much more useful, and flying a precise rate 1 turn is anyway trivial with a horizon (usually 20-22 degrees at a typical IFR tourer cruise speed), and the only time one might be flying precise rate 1 turns is when doing timed turns which is an archaic procedure beloved in partial panel flying (I did most of my FAA IR on timed turns) which one would almost never do in reality, with a battery powered GPS available.

The AI and T&S are not capable of doing each other's jobs. Their gyroscopes are oriented on different planes, and each not capable of sensing motion in the other's sensative axis.

You will get varying opinons as to which is the preferred instrument to fly with, if you only have the one gyro. I certainly don't discount the turn and bank for getting you where you planned to go, in the absense of all other gyro instruments.

I flew an MD500 helicopter 1900 miles across Alaska and British Columbia. It had no gyro instruments. It was the T&B I felt I missed the most.

So which would you prefer IF you could only have an AI or a T&S?

I'd take the AI, I mean a rate one turn can be worked out with a relatively simple calculation if one didn't have the turn co-ordinator but an AI tells you everything you need to know really about what the aircraft is doing.

That said, I wouldn't do away with the turn co-ordinator/turn and slip in favour of a back-up AI, it would make more sense to keep it and add an AI somewhere else in the panel.

G1000 system of course if one loses the AHRS or the screen you are left with the back-up AI and no turn co-ordinator, which I'd much rather have than just a turn co-ordinator.

Ajstoner 21

All the red flag on the typical GA turn cordinator tells you is the instrument is recieving power. Since most failures are caused by a failure of the instrument gyro motor the flag will still disappear because while the instrument is recieving power but the instrument will not be functioning because the gyro is not spinning up.

As was pointed out (before the thread diversion into the legalities of back up instruments for IFR flight operations ) a functioning turn Coordinator is not required in the USA for day VFR flight, but if it is installed it must be working.
The likelyhood a failed turn coordinator would actually cause a problem is extremely low, but not zero. The best way to handle this is to have an A & P
have a quick look at the instrument, as the problem could be as simple as a loose connection, and if he/she finds nothing obviously wrong pull and collar the circuit breaker and placard the instrument as U/S.

From your intial post I assume you do not own the aircraft. If that is the case your only duty is to inform the owner of the problem. If you do own the aircraft and assuming you are not flying it IFR than I would shop around for the cheapest exchange overhaul from one of the big instrument repair outfits.
You can send in your unit for repair but usually it is cheaper and faster to just get an exchange instrument.

Not sure in Europe but in North America a AI can replace a T & B/turn coordinator provided it is driven off a different power source than the primary AI instrument and has a slip/skid ball incorporated into the instrument. Several companies make electrically driven AI's with a little curved tube style ball built into the base of the instrument so that it can be used as a replacement to the T & B

Actually, Richard Collins did very champion replacing the turn and bank (or turn coordinator, as the case may be) with a second attitude gyro. His wasn't simply a recommendation to install another attitude gyro, but he strongly felt that the 2nd attitude gyro presented better information than the TBI. Especially in the case of a failure involving loss of of the primary attitude indicator.

Not so much that it was adequate, but that it was part of the basic instrument package from the earliest days of instrument flying. This goes back to the days of compass, ball, and airspeed, with the turn and bank indicator being an additional means of setting establishing a heading.

At the risk of some having a fit because I invoke an example from a more complex aircraft, it's worth noting that in more complex aircraft we seldom if ever teach, train, or use timed turns. Instead, we handle instrument failures with backup instruments. In a light airplane, this becomes the partial panel world, and it makes sense to resort to timed turns if one has no heading information and is down to the TC or TBI. Fair enough. Having a backup reference powered by a different source, however, is an even better idea.

The problem with the attitude indicator is that while it does provide better overall awareness of one's attitude (try flying partial panel off a turn coordinator in turbulence, while IMC), it does't give actual turn information or turn rate information to enhance one's ability to stay on a heading.

One can still do a very credible job with an attitude indicator, however. For timed turns, using a specific bank angle is closer to the way it's done in complex or advanced aircraft, anyway.

A really handy technique that works in a light airplane is to use the attitude indicator to make corrections. I used to do a job that had an observer requiring slight corrections to heading, and the observer might say "give me 20 left." Snap to a 20 degree left bank, hold for three seconds, and go wings level. It's very close. Same for a 30 degree course correction, or a 10 degree course correction.

I'm sure there are those out there that can fly a 1 degree course of the magnetic compass in turbulence at night in a cloud...but I'm definitely not one of them. I'm not one who has found the magic of being able to fly an accurate heading on the TBI alone in turbulent, IMC, partial panel conditions. I find that use of the attitude indicator enhances awareness substantially.

This may all be a mute point (and I mean mute, as in silent, rather than moot) in a Cessna 172, where one tends to do not a lot of hard IMC work (or shouldn't, really). It's a worthy discussion of instrument flying in general, however, which is a good byproduct of thread drift.

This is the whole point of field approvals for major repairs or alterations, as well as supplemental type certificates.

I'd rather have the attitude indicator. I spend more time in large aircraft lately than small aircraft, but I'm more interested in attitude awareness,and most turns I do are at 25 degrees of bank (same as what the flight director provides).
A second attitude indicator does indeed conform to those requirements. However, those requirements are for certification, not continued airworthiness thereafter, and installation of a second attitude indicator is not restricted or changed by the requirements of 14 CFR 23.1321. Especially not when an existing attitude indicator is already in place.

No. The inoperative instruments or equipment must not only be placarded, but deactivated in an acceptable manner. One needs to know the system in use and check with the current maintenance publications to know if and how an item can be removed or deactivated. In the even the item is removed, which is advisable in some cases but not all, the weight and balance documentation must also be ammended.

Pilot DAR already addressed this, but no, pitot-static systems aren't self sealing. If you do open the system, you also invalidate the IFR certification of the system, and a complete test and certification of the pitot-static system must be performed again before the aircraft can be certified once more for IFR flight.

To open these systems up requires mechanic certification (or "engineer" certification, if you will), and should be left only to those trained to work on the airframe. Improper handling of the components can introduce all kinds of problems. I've experienced a complete instrument loss on two occasions in airplanes in which someone had opened the pitot-static system and not capped the lines while work was being done. Insects entered the system, and died. In IMC, the insects apparently swelled and plugged the system off, causing all kinds of havoc. They were in various parts of the system, too, causing unusual failures that didn't conform to what one might normally expect from a blocked pitot tube, or blocked static source.

I think you may have misunderstood my point. When I said that the second attitude indicator provides more useful information, I wasn't referring to it augmenting the primary attitude indicator. You're correct that it doesn't provide anything extra over the existing attitude indicator. It does, however, provide a lot more useful data if the primary attitude indicator fails, than what the turn coordinator can do for maintaining control of the airplane.

My modest few hundred hours of actual IFR, and several instrument failures during that time, entitle me to an opinion, but certainly not the last word on the subject. I will leave that to those with much more experience on instruments.

If I were being asked to issue STC approval of an instrument panel layout which did not conform to the design standard, I would be referring the proposed design back to Transport Canada engineering department for review. I agree that a backup AI is great, but if the trade off is loosing the only instrument which actually indicates a turn, the cost seems too big to me.

If the subject aircraft is going to fly enough actual IFR that the affect of instrument failure is really a concern, it should be equipped with redundent systems anyway, then there would already be a back up AI and T&B and the problem has gone away!

The Wiskey compass gives a turn indication.

A turn and bank indicator won't keep you right side up in a turbulence and a rough flight. By the time you react to a turn in one direction and correct the other, without any other means of keeping wings level, you can be upside down in some airplanes. A 300 or 400 series Cessna with full tip tanks and no additional instrumentation can turn ugly when a pilot gets into a roll oriented pilot induced oscillation on partial panel.

There was a time when a turn and bank was state of the art. That time was a very long time ago. It's a useful instrument, but far less useful than an an attitude indicator when one has to go partial panel.

I was looking for an answer to my question in context of a failed VSI or maybe a backup altimeter. Suppose my VSI is showing a defect, say, reading +100 ft/sec while on the ground. I would normally note this, maybe tape a piece of paper over it to avoid confusion, and that's it. But now you're telling me that I should deactivate this instrument, which would require me to dive behind the panel and start messing about with the pitot/static system (actually the static only, but you get the point) - which I'm not allowed to do (and rightfully so, I'm not arguing that).

So even though a VSI is not required for VFR flight, if it shows a defect it has to be disconnected, not just placarded U/S. And I'm not allowed to disconnect it myself so in effect it has just turned into a no-go item, right?

Backpacker

I dont think that is quite correct. It is not uncommon to see a VSI or tac placarded to indicate over reads / under reads by x, whereas an altimeter must be within tolerance (for obvious reasons). Someone more knowledge that I will need to clarrify where matters stand on these issues so far as the FAA are concerned.

The inaccurate VSI situation has several possible outcomes:

Pilot decides he cannot safly fly the aircraft in that condition, and writes it up for repair before further flight = Outcome; did not go flying that day.

Pilot flies the aircraft as is, and so do the next pilots = Outcome; it never gets fixed.

Pilot goes flying that day, then tapes across the instrument "U/S", and snags it upon landing = Outcome; the next pilot decides he can still fly his next flight safely, and it probably gets fixed, or at least a error card, at the end of the day's flying.

Pilot (who is not a mechainic) decides to perform "do it yourself" maintenance under the panel = Outcome, he gets caught, and gets a bill for a complete inspection of everything under there which he could have affected, and the authorities discuss enforcement action for unauthorized maintenance, or; his faulty work goes initally un-noticed, then at the time of the accident investigation later, the unauthorized maintentance is discoverd to be a contributing factor. Very not good.

As a pilot, you have some latitude to decide to fly with a known defect, which will not affect the safety of the flight. You my just have to defend it later if challenged. If you have any latitude to go fixing the plane, you are formally qualified for maintenance, and you know exactly what privilages you have.

If you want the standards of servicability for an aircraft in black and white, you'll need to fly an aircraft with a published minimum equipment list, then there's no doubt.

Okay, I've been re-reading post #7 and did some parsing to determine the hierarchy in § 91.213. I thought I had to conform to this bit:

So.. I thought I had to deactivate the VSI, which would require maintenance privileges to disconnect the static tube.

But upon reading the article a bit closer, the following works as well:

Sorted.:ok:

Backpacker, first of all, I did NOT tell you to "dive behind the panel and start messing about." Nothing of the kind.

You need to determine if indeed you have a problem. It's very common for the VSI in light airplanes to be indicating a slight climb or descent on the ground. This doesn't necessarily indicate a "defect." In fact, any instrument training handbook I've ever seen states that one should note the difference between the reading on the ground and zero, and apply it in flight. Thus, if the VSI is indicating -100 fpm, then in flight if one intends to establish a 500 fpm descent, one uses the -600 fpm marking on the gauge. Simple enough.

If indeed you do have a static problem, then you need to determine if it's affecting the rest of your system. You're quite correct that you can't fly with inoperative instruments or equipment unless it's in a condition that makes the aircraft "properly altered." You can accomplish this in four ways: a "ferry permit" (special flight permit); applying a Minimum Equipment List (MEL); Deactivating or placarding as required per 91.213; the use of an STC or other supporting paperwork which changes the certification and airworthiness documentation for the aircraft. In each of these conditions, a legal means is made to alter the airplane and it's type certification.

If your VSI has a problem, your altimeter may have a problem. As I said before, you need to look carefully at what's going on and determine if it presents a hazard to flight. You may not be qualified to make that determination; you may need the help of a mechanic to make the determination. Ultimately it's up to you to take the responsibility for the airworthiness every time you get in an aircraft and put it in service, but part of your responsibility is also to defer to qualified individuals in determining the status of a problem or condition.

A day VFR flight, you may be able to go fly without instruments that aren't required for day VFR flight...so long as they're in a condition which meets the requirements of the regulation.

I need to again include the caveat that I don't work with the CAA for certification or maintenance, and can't speak to regulation in the UK, or to JAA maintenance regulation, specifically. Therefore, you may see some difference with local regulation and policy. That part, you'll have to ferret out for yourself. We've discussed 91.213 of the US Federal Code of Regulations, however, and that part I can address.

When you placard a part of a system, you may be unwittingly placarding the entire system. The static system is interconnected; a problem with the system affects multiple instruments. In some aircraft, it can affect dozens of other components and systems. In the airplane I'm flying at the moment, for example, a pitot blockage can affect nearly 30 other items. Static problems affect fewer items, but still have big impacts on what I can and can't do with the airplane, legally (and safely). Even big airplanes with redundant systems have crashed as a result of pitot and static blockages. Clearly it can be serious.

Back to day, VFR: if indeed there is a static system problem (how do you tell it's just the VSI, or that the VSI is actually failed?), then if you're making a determination about something being inoperative, it's actually the static system which is inoperative. This affects more than the VSI, and now you're having to consider instruments that are required for day, VFR flight. Now the airplane can't fly day VFR. You see the point?

All me an example, if you will. As a new pilot in the PB4Y-2 (a version of the B-24), a WWII bomber with dirt-simple instrumentation, very much like a 172, I was sent on a fire dispatch in Florida. The airplane had been sitting in the rain. Heavy rain, in fact. As we taxied out, during execution of the checklist, we noted a discrepancy in the altimeters. Not a big deal, we thought, as we'll not be climbing above 500' anyway, during the entire flight. In fact, the entire flight would be conducted as much by "feel" as by instrument work, due to the low altitude, contact nature of the flying.

Bear in mind that these kind of flights were very quick; five minutes from notification to airborne, in many cases. Often quick enough that one didn't always have the coordinates of the destination when departing; the dispatch might be simply "takeoff and fly south." We might get the destination once in the air; it was firefighting, and time was often a very important consideration. This isn't something that one has to worry about with a weekend rental or pleasure flight, or a condition one shouldn't impose on one's self during a typical private flight, anyway.

On the takeoff roll, I was head down making adjustments to the propellers through a series of switches on the center pedestle. The nose of the airplane was too long to see over, especially once it had been raised, so to see out one had to sit up, stick one's head in a bubble window to the side, and look forward, with a very limited view. Being very new, I was trying to get everything done, perhaps a little behind the curve, so to speak, and was surprised to hear the captain yell "Rotate!" I thought I'd missed my call, and glanced at my airspeed indicator.

Rotation speed was 95 knots, but when I glanced at my airspeed indicator, I saw barely 70 knots. I felt the wheels skipping slightly, just the same as one might in a light airplane when it's ready to fly. The 4Y was flown very much like an oversize light airplane. I felt the captain rotate, and again felt the airplane skip slightly like it was trying to fly, and shortly thereafter the nose slammed back down. That got my attention.

I looked up, put my head in the bubble and saw nothing but trees. We were nearing the end of the runway. I broke the safety wire on the emergency jettison switches, and prepared to blow the tank doors. The captain rotated hard and we were airborne, but went through the trees at the end. We found ourselves in a real fight; a struggle to stay airborne. The captain called for flaps up; this was normally accomplished at 130 knots, but when I checked my airspeed, it was just coming through 95...we were just at rotation speed. I yelled "unable!" Raising the flaps on that airplane made a 50 knot difference in stall speed, and we were still at rooftop level. You can imagine, if you've ever retracted flaps just after takeoff and started sinking, the possible outcome. We were also heavy, at 80,000 lbs.

At that point we were both heads outside, in the bubbles, calling out powerlines, structures, obstacles, because we were going around them, not over. Very, very gradually we climbed. The captain yelled "I don't understand it. I'm over 130 knots; we should be flying!" I yelled back (it was a very loud airplane inside; most communication was done with hand signals) that I was showing 95. We looked at each other and the lightbulb finally went on. "Alternate Static!" The captain opened his, I opened mine, and his sytems quickly dropped back to match mine. He had followed errant indications, had attempted to rotate early, and the nose-high, early rotation delayed our lift off, as well as killed climb performance as we ended up airborne behind the power curve, low, and slow.

There's more to the story, but the point is, drawn out though it may be, one shouldn't make assumptions about instrument error,and what else may be affected by the instrument error. It may not be a simple busted VSI; you may have a static problem, or additional problems. I've seen a number of cases of static blockage that included pitot blockage, too. In fact, I flew in one location where leaf-cutter bees could plug up or block any port in the airplane, including fuel drains, in an amazingly short period of time; where there's smoke, there's often also fire...you may find more than one blockage, or blockage at more than one point in the system.

In the case of the Turn and Bank mentioned earlier, it's well to remember that the problem may be a short in the wiring, not just the instrument. One could placard the instrument, and then still end up with a fire. One needs to make sure the problem is fixed. I saw a Cessna 150 catch fire once because the cigarette-lighter adapter (DC adapter receptacle) in the airplane that was placarded inoperative was a little more than simply inoperatve.

Point is, there's more to the picture. Or could be.

No, not entirely.

As with most regulation, looking in one place usually won't cut it. There's nearly always more to the big picture.

Let's say you elect to fly with the inoperative VSI. You summon a mechanic, but without opening the static system and blowing out the lines, and putting the VSI on a test bench, the mechanic can't do too much. We've already noted some potential implications of a static or pitot (or both) failure; I'm sure you can think of more. The mechanic glances over the system, and says he doesn't see anything wrong (In the scenario involving the PB4Y, both pilots were A&P mechanics, as well). You put an inoperative sticker on the instrument and go fly. Sounds simple enough. If the aircraft has a squawk sheet or flight maintenance record it should reflect the change.

The mechanic can't do too much with the system, and it isn't self-sealing from one static instrument to the other, because the one instrument can affect another (remember that the alternate static relief in some aircraft is to break the glass on the VSI). You may be operating an unairworthy airplane, and to take the US regulation a bit farther...if the paperwork isn't complete and the aircraft isn't in a properly altered condition, then the airworthiness certificate is invalidated; you're now flying an airplane without a legal airworthiness certificate.

The regulation states that either the pilot, or "a person appropriately rated to perform maintenance" must determine that the aircraft is airworthy. Remember that you're held accountable for the determination. "It looks good," isn't really a determination, nor is "I think it's okay." How, as PIC, will you know the extent of the problem, or whether you have a VSI issue or a static system issue? If you have a static system issue, you'll definitely be flying an unairworthy airplane.

This also applies to the original scenario. The TC or TBI is placarded inoperative. A mechanic is summoned, and he gets behind the panel to disconnect the cannon plug on the instrument. This done, you apply the placard, and off you go. To be legal, reference needs to be made to Part 43, covering maintenance, which will address, among other things, maintenance records. Again, if the maintenance records aren't ammended to show these changes, then the airworthiness certificate is invalidated, the airplane is made unairworthy, and one can be violated (the subject of enforcement action) for flying both an unairworthy aircraft, and one without a valid airworthiness certificate. Simply slapping a piece of tape on there with an "INOP" sticker is often not enough.

Not to belabor a long post any more, but the standard for airworthiness should be briefly addressed. Airworthiness is a two-part concept; the aircraft must be legal, and it must be safe. We mostly understand the safe part; it's what we're looking for on a preflight, and inflight. The safe part may involve things we can't see, however, and we need to be careful to consider that what we think is the problem, may simply be the tip of the iceberg; it may be something else entirely. That slight engine roughness might simply be carburetor ice, but may be more; it may be oil fouling a spark plug, it may be a valve problem, it may be a timing problem. When determining what's safe, it's important not to overlook something and allow an unsafe condition to exist.

I once dealt with a Piper Seneca which pilots reported made a popping noise with the second "notch" of flaps in the traffic pattern. It didn't do this on the ground, but in flight, and most who flew it guessed that it had a bad pulley in the cable system for the flaps. It turned out to be a spar that was cracked in three places, grinding and popping against itself as it shifted position along the break, when the flaps were applied under a load. Given more time, I've little doubt that it would have failed in flight and shed a wing. Be careful not to make the wrong determination.

The other part, or second part of airworthiness is the legality. Unless the airplane is in full compliance with it's type certification, then it's not airworthy. An airplane can be properly altered to conform with additional documentation and be legal, such as conformance with a supplemental type certificate. This supplements the original type certificate, modifying it. Likewise, compliance with 91.213 meets the requirement to be "properly altered in a manner acceptable to the administrator" (FAA considerations here, again)...but one has to be careful that one is actually performing according to 91.213...which by default also includes Part 43, and in some cases additional regulation.

Doesn't anyone just whip out their Swiss Army Knife and adjust the zeroing screw? Or is that not allowed unless you are an AME?

Doesn't anyone just whip out their Swiss Army Knife and adjust the zeroing screw?

Absolutely, but then if my T & S fails I get my tool kit out and try to fix that as well.

Rod1

People probably do, but no, it's not allowed unless one is an appropriately rated person authorized to perform maintenance (eg, engineer/mechanic).

Then again, if you do have a problem other than an instrument requiring a slight adjustment, you may have just disguised, or hidden the problem.

Guppy did you work out what had caused the problem other than the obvious that it was some sort of static fault? How did the altimeters and VSI behave during the flight?

Do you mean for the 4Y flight with the erratic instruments on takeoff?

We did find the problem. Externally, nothing appeared amiss. On the ground on return, everything was matched up and working.

The takeoff involved an overrotation in an effort to get airborne before striking the trees, and in so doing, the captain struck the tail. Not the tail itself, but a tailskid that had once been retractable, but that was permanently welded down, in the airplane. It had a thick magnesium skid plate on the bottom. When that struck, the skid was thrust up into the belly of the airplane, and the magnesium ignited. It produced a fireball that enveloped the tail of the airplane, and ended up burning off the last 25' or so of the underside of the airplane...skin, stringers, etc.

The problem was caused by rain. The 4Y had fairly large static ports. Plugs were placed in the ports, but the plugs still allowed water, due to the design of the ports themselves. Because of the nature of the flying, pitot covers and static plugs, and other security devices were removed during the morning preflight. When heavy rains came, with strong winds, the rain was nearly horizontal, and entered both the pitot tubes and the static ports quite handily, as we later determined.

The cockpit indications were fairly nill, but weren't symmetrical or uniform throughout the system. In other words, what appeared to be an altimeter mismatch was part of a bigger problem.

We broke the static system in multiple places and blew the lines out through filters. Water was the culprit in some points, and low areas in the line, and in others we found a dead insect which swelled in the presence of water.

We also instituted a policy of keeping everything covered until the dispatch itself...at which time one person would remove and stow all the covers.

Are you sure you were not hit by an RPG?

I routinely machine magnesium components in my workshop... made various things for my TB20 out of it. Lightweight conrods really do up the power... Anyway, lots of "experts" told me that the stuff catches fire really easily when being machined.

Well, being really a big kid who loves setting things on fire, I tried to set fire to some magnesium.

It's amazingly hard to do, especially with a thick section. You have to get a blowlamp (an oxy-acetylene welding torch works better but I don't like wasting the gas) and you have to get at least a corner of the component bright red hot. With a hi-temp blowlamp (the one which uses a propane-acetylene mixture; yellow bottles rather than blue bottles) it takes about a minute to set fire to a piece of magnesium.

It's quite impressive after that, though...