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Archive for the ‘Maintenance’ Category

Has this ever happened to you?  Sometimes, you just look at a mechanical part and think, “I wonder what’s inside that?”  or maybe you ask yourself,  ”How does this thing exactly work?”  Pilots by nature I believe are pretty inquisitive creatures.  For us, surface or book knowledge alone does not suffice.  We want to personally know and understand what makes things things “tick”.

I had the privilege recently of helping an aircraft mechanic as he took apart an alitmeter.    Why did we need to take it apart? Well, we had no good reason to take apart the altimeter, we just wanted to.    The really impressive thing here is not that we took it apart…but that we got it back together!  And it works….well, sort of.

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Do you know your airplane's MPG?

“What is meant by the term ’specific range’?”

This is the question that I asked David Wyndam of Conklin and de Decker after coming across this term while looking at some aircraft comparison performance tables.

The performance tables listed the specific range for various airplanes in both long range and high speed cruise.  For instance, the specific range for a Cirrus SR22 was listed as 2.051 for long range cruise and 1.810 for high speed cruise.  But what exactly do those numbers mean and what formula was used to devise those values ?

Specific range is the distance an aircraft travels per unit of fuel consumed. Typically this is expressed in nautical mile per pound of fuel.  So for instance, the Cirrus SR22 has a high speed cruise specific range of 1.890.  That means for every pound of fuel used, the Cirrus SR22 could fly 1.890 nautical miles.   Here is another example.  The Beechcraft Premier that I fly has a SFC of 0.557 NM/lb.   If 1 lb will take me 0.557 NM that means with 1,000 lbs of fuel the Premier can fly 557 NM.  This is the aviation equivalent of MPG.

One purpose of SFC is to help determine the efficiency of an aircraft.  This can be used when comparing aircraft for purchase.  Fuel efficiency is important for knowing what your direct operating costs will be as fuel is one of the largest components of DOC.

Now we don’t typically use pounds for the fuel unit in piston aircraft, we use gallons.  We also know that 1 gallon of 100LL weighs approximately 6 pounds.  So if the Cirrus can go 1.890 nautical miles on 1 lb, that mean on 6 lbs (or 1 gallon) the Cirrus can fly 11.34 NM.   On 60 lbs, or 10 gallons, the Cirrus SR22 at high speed cruise can fly 113.4 NM.  Here is the high speed specific range cruise figures (and some corresponding ranges)  for 3 different single-engine piston aircraft:

To find the values in the lower row I took the specific ranges values and multiplied them times 60 to find the range on 10 gallons of fuel.  Here is a map with range rings representing those values:

How far can you go on 10 gallons?

So how can you figure the specific range (SFC) for your aicraft?  It’s actually quite simple.  Just take your TAS (true airspeed) and divide it by your fuel flow in pounds.  So, let’s say your TAS is 130 knots and you are burning 11 gallons  an  hour (or 66 pounds an hour):That is a specific range of 1.969.  On 10 gallons of fuel, this airplane could fly 118 nautical miles.

So is the higher SFC number always the more efficient airplane?  Not always.  For instance, the Boeing 737 has a long range SFC of 0.090 and a CJ3 has a SFC of 0.606.  But, if  a 737 can carry 135 people and the CJ3 can only carry 7, which aicraft is really the more efficient airplane?  There are many other factors to include in efficiency comparisons as well such as fixed and non-fixed maintenance costs.

Thanks again to Conklin and de Decker for their helpful input to this post.  If you are looking for professional aicraft comparison software or handbooks, Conklin and de Decker have a wide range of products for a wide variety of aircraft both single engine, jet and rotorcraft .

Here is a very important question that came in from Travis:

I’m training in C172’s from the early-mid 80’s, and I’ve got a mag check question: Occasionally when I do the mag check, I’ll get an RPM drop on one mag, but not on the other. The engine always runs fine, just no RPM drop on one mag where there was on the other. What’s going on here? I’ve never had a problem, but now I’m just curious! Thanks!

Travis, what you are describing sounds like you have a “hot” mag.  Basically a magneto that most likely has a broken P-lead (it could also be the mag. switch).  The P-lead goes between the start switch and the magneto.  The p-lead functions to ground the magneto (turn it off).   You need to report this to a A&P mechanic as soon as possible because this is a dangerous situation.  If someone happened to turn the propeller by hand, it could conceivably start the engine while sitting on the ground or in the hangar (ouch!).

When conducting a magneto check, you always want to see an RPM drop of some kind.  A lack of that drop, indicates the magneto is not shutting off.  Another way to check this is while at low RPM (<1000) you can quickly turn all mags off and the engine should behave like it is going to quit, if it keeps on running, this is another sign of a broken magneto wire (or magneto swtich).

P-lead on an magneto

John writes asking about the W in the A.R.R.O.W. acronym:

With regards to the acronym “ARROW” for required documents: Where in the FARs is the specific requirement for the “W”? I realize the requirement would be indirectly there because of the requirement for the AFM. But the acronym would be rather lengthy if we listed all the required components of the AFM.

If you’re reading this post, and haven’t started flying yet, here is something you should know about aviation: we have a acronym and mnemonic for EVERYTHING.  Especially when it comes to remembering lengthy regulations.  In this case, ARROW is the acronym used to help remember the required basic documents and paperwork that must be on board every airplane to be legal.   To review:

A – Airworthiness Certificate

R – Registration

R – Radio Station License (not required in the U.S.)

O – Pilot Operating Handbook (specifically the Operating Limitations)

W – Weight and Balance

John’s question is simply, “Can you show me in the regs where it says we need the W?”  Sure John, I’d be happy to help.  I do want to point out though that acronymns and mnemonics are not officially recognized.  They are not endorsed by the FAA.  We use them in the training community simply as a memory aid and device.  Maybe sometime, a long time ago, some instructor decided that “ARO” wasn’t as easy to remember as “ARROW”.

There are actually a couple of places where  we can find this requirement though.  The most common one most instructors point to is…

1) FAR 91.103 – Preflight Action.

This regulation details the information that pilots are supposed to become familiar with  before each and every flight.  Of course, we have another mnemonic (RAWFAT) to help us remember the these requirements:

(I have place an asterisk by the ones that are only required for flights not in the immediate vicinity of the airport)

R- Runway lengths (every flight)

A – Alternates *

W – Weather *

F- Fuel requirements  *

A – ATC delays *

T- Takeoff/landing distance data (every flight)

Let’s look at the last one – T.  If you are required to know your takeoff / landing distance data for each and every flight, it is somewhat implied that you are going to have to know your weight and balance as your performance will vary, obviously, with changes in the aircraft’s gross weight.  In fact, 91.103(b)(2) states that if for some reason you are flying an older airplane, for which there is not the standard  takeoff and landing data tables than:

(2) For civil aircraft other than those specified in paragraph (b)(1) of this section, other reliable information appropriate to the aircraft, relating to aircraft performance under expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature.

2) FAR 91.9 – Civil aircraft flight manual, marking, and placard requirements.

This regulation states pretty clearly that yes, we need an approved airplane flight manual but also:

..no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual…

In every limitation section of a Airplane Flight Manual (or POH) you will find a section detailing not only the max. and min. weight limits but also the center of gravity limits.  So again, this implies that if you are required to operate within these limitations than you will have to complete a weight-and-balance calculation prior to every flight.

3) FAR 43.5 – Approval for return to service after maintenance, preventive maintenance, rebuilding, or alteration.

This regulation states that if any major repair work has been done on your airplane and that repair work results in a change or alteration of the limitations of the airplane.  That information must be entered in the airplane flight manual (AFM).  That is why you see all those updated weight and balance forms in the aircraft’s AFM.  It’s required.

I hope this has helped answer your question about why the “W” is in “ARROW”.

Fly Safe.

Here’s the question I received this morning from Jorge:

Hi could you please tell me if the alternator handles all of the electrical load?

It would have been easy to answer this question (yes) but I thought it would be fun to use a flash animation developed by MSAviation with some narration to demonstrate this point.  Here is your answer in full:

If you would like some more information about the flash animation used, or would like a copy of your own.  You can purchase aviation flash animations for pilots and flash animations for flight instructors using this link.  Aviation Flash Animations

Here’s a question that was recently sent to me:

It has happened to me twice recenely. I fly a Cessna 172R (1997). Under very normal weather conditions at low altitudes like 2,000 to 3,000 MSL, the RPM ramps up very high getting into the red zone and I have to pull the throttle back to almost half position to get it down to maybe 2,300 RPM.

First, my disclaimer: I am not an A&P mechanic.  However, I did stay at a Holiday Inn Express last night so…

No seriously, I’m not sure what is going on here.  This is a unusual problem to say the least.  I ran the problem by my friend who happens to be a IA with some 172 experience and he thinks that you may have a problem with a clogged fuel injector but he is merely guessing.  It might be that you are only operating on 3 cylinders and suddenly the fourth injector becomes unclogged and comes firing away.  If it has happened twice, you definitely need to take your airplane to a trusted A&P.  If it is a rented airplane than you should fill out the appropriate squawk form and report the malfunction.  I am very curious as to what the problem might be.  Be sure to drop me a note when you find out what is causing this.  Sorry I can’t be of more help here.

Fly Safe!