Diagnose this ...

Recommended Posts

OK. Not one of you has provided a systematic scheme for working out what is going on here, and, despite my saying that knowledge of aircraft systems would solve the problem, the best that people have come up with is a faulty ASI.

Here's the original data, so let's go through it:

It's Sunday morning and you and your "significant other" decide to take a flight to get a \$100 hamburger at a beach side airport. You check the weather and find that your intended flight path is dominated by a strong high pressure cell of up to 1024 hPa and winds are light and variable below 5000. Temperature is in the low 20's; humidity in the 40's and dew point is below 10C.

You take off and set for cruise, checking that you have "Carby - Cold; Friction Nut - firm; fuel On and sufficient quantity; oil pressure and temp "in the green"; RPM set to 75% cruise power. The aircraft is trimmed nicely. The needle of the VSI isn't moving away from Zero. The arrow of the ASI points directly at your desired value.  God's in His heaven and all's well with the world.

1.  The flight will be through a very strong High pressure area within which winds will be light and variable. Light and variable means there's no meaningful wind speed or direction.  It will also mean that the aircraft will be very stable - nothing to knock it off straight and level flight.

2.  The air temperature is nicely above freezing level and humidity is low, so you won't need to constantly watch for carby icing.

3.  At Top of Climb you have established the aircraft on course and in straight and level flight with engine power set.

4.  You fly along for 30 minutes, watching the scenery and notice nothing untoward. However, after 30 minutes of smooth flight you determine that, while you are on your intended track, you are behind your ETA.

The flight has been smooth and you are on track, but late. That could indicate a head wind, but wind doesn't blow in a steady stream. Any wind acting on the aircraft would have caused some degree of buffeting. You dismiss the head wind theory and look elsewhere.

You scan the instruments and see that the ASI is reading less than what you expected. What can cause this? It could be a problem in the pitot system, or it could be reduced thrust.

The simplest reason is that the engine is not producing the thrust normally expected at a certain throttle setting. Then take a look at the tachometer. If it is showing a reading lower than what was set at top of climb, there could be a problem with the engine.

Three things could be the simple cause of that. Check are the throttle friction nut. If the nut has wound itself out, the throttle rod could have walked backwards, reducing power. Check that the nut is tight. Likewise the mixture control could have wound itself out. Check that. the carburettor heat could have started to bring itself on. Check that.

To run an engine, you need two things - an air/fuel mixture and a spark. You've checked the things that affect the air/fuel mixture (the assumption is that since the flight has been smooth and unremarkable, that the fuel supply is OK). That only leaves the spark.

Remember when you did the pre-takeoff engine checks? You ran the engine to 1500 RPM, or whatever fast idle was for the engine, then you checked the magnetos, carefully noting the drop in revs as you switched from Both to Left of Right. At fast idle we expect a rev drop whose size is dependent on the engine.

A 75% cruise power setting results in engine RPM well above those of fast idle.  If shutting off a magneto at, say 1500 RPM gives a 50 RPM drop, what sort of RPM drop would you get at the higher RPM of 75% cruise? That RPM drop would result in less thrust, resulting in a lower air speed and hence a lower ground speed. That's could be why you are behind ETA. One of the magnetos has failed.

Remember I said that this diagnosis depended on your knowledge of aircraft systems.

I think I'd accept that I had a failed magneto, in accordance with Occam's Razor. The redundancy in the ignition system gives me faith that I can complete my flight, but maintain greater vigilance, and I'll look at the magnetos after I land at the beach side airport while my significant other is having a hamburger.

• Replies 153
• Created

Popular Posts

It has been good to see that the responses are generally the same as the path I took. While working on it, I knocked the exhaust pipe with my knee and thought I heard a faint rattle. I gave it a

In the real world the pilot would assume that there is a head wind and no diagnosis would happen.

Left the wheel chocks attached to the U/C?

Posted Images

Doesn't compute on my engine, Fairbanks Morse, mag, BUT Single plug heads.

So it would stop completely.

spacesailor

Share on other sites
21 minutes ago, old man emu said:

OK. Not one of you has provided a systematic scheme for working out what is going on here, and, despite my saying that knowledge of aircraft systems would solve the problem, the best that people have come up with is a faulty ASI.

Here's the original data, so let's go through it:

It's Sunday morning and you and your "significant other" decide to take a flight to get a \$100 hamburger at a beach side airport. You check the weather and find that your intended flight path is dominated by a strong high pressure cell of up to 1024 hPa and winds are light and variable below 5000. Temperature is in the low 20's; humidity in the 40's and dew point is below 10C.

You take off and set for cruise, checking that you have "Carby - Cold; Friction Nut - firm; fuel On and sufficient quantity; oil pressure and temp "in the green"; RPM set to 75% cruise power. The aircraft is trimmed nicely. The needle of the VSI isn't moving away from Zero. The arrow of the ASI points directly at your desired value.  God's in His heaven and all's well with the world.

1.  The flight will be through a very strong High pressure area within which winds will be light and variable. Light and variable means there's no meaningful wind speed or direction.  It will also mean that the aircraft will be very stable - nothing to knock it off straight and level flight.

2.  The air temperature is nicely above freezing level and humidity is low, so you won't need to constantly watch for carby icing.

3.  At Top of Climb you have established the aircraft on course and in straight and level flight with engine power set.

4.  You fly along for 30 minutes, watching the scenery and notice nothing untoward. However, after 30 minutes of smooth flight you determine that, while you are on your intended track, you are behind your ETA.

The flight has been smooth and you are on track, but late. That could indicate a head wind, but wind doesn't blow in a steady stream. Any wind acting on the aircraft would have caused some degree of buffeting. You dismiss the head wind theory and look elsewhere.

You scan the instruments and see that the ASI is reading less than what you expected. What can cause this? It could be a problem in the pitot system, or it could be reduced thrust.

The simplest reason is that the engine is not producing the thrust normally expected at a certain throttle setting. Then take a look at the tachometer. If it is showing a reading lower than what was set at top of climb, there could be a problem with the engine.

Three things could be the simple cause of that. Check are the throttle friction nut. If the nut has wound itself out, the throttle rod could have walked backwards, reducing power. Check that the nut is tight. Likewise the mixture control could have wound itself out. Check that. the carburettor heat could have started to bring itself on. Check that.

To run an engine, you need two things - an air/fuel mixture and a spark. You've checked the things that affect the air/fuel mixture (the assumption is that since the flight has been smooth and unremarkable, that the fuel supply is OK). That only leaves the spark.

Remember when you did the pre-takeoff engine checks? You ran the engine to 1500 RPM, or whatever fast idle was for the engine, then you checked the magnetos, carefully noting the drop in revs as you switched from Both to Left of Right. At fast idle we expect a rev drop whose size is dependent on the engine.

A 75% cruise power setting results in engine RPM well above those of fast idle.  If shutting off a magneto at, say 1500 RPM gives a 50 RPM drop, what sort of RPM drop would you get at the higher RPM of 75% cruise? That RPM drop would result in less thrust, resulting in a lower air speed and hence a lower ground speed. That's could be why you are behind ETA. One of the magnetos has failed.

Remember I said that this diagnosis depended on your knowledge of aircraft systems.

I think I'd accept that I had a failed magneto, in accordance with Occam's Razor. The redundancy in the ignition system gives me faith that I can complete my flight, but maintain greater vigilance, and I'll look at the magnetos after I land at the beach side airport while my significant other is having a hamburger.

I was going to make no further comment but, you've demonstrated clearly your own lack of understanding....You stated that the ASI was reading exactly where it should and that the ground speed was slightly less than than anticipated, which rules out the engine straight away. Then in your explanation you tell us that you have a lower airspeed....contradicting what was said previously. This is nothing to do with Occam's Razor, and there would have been a more clues than a couple of knots of missing ground speed.

Next time I'll know better and not bother.

• 3
• 1
Share on other sites

You have now presented more things wrong than a simple ground speed lower than planned which was stated in the question. Are you sure you aren't now employed by CASA?

• 2
Share on other sites

In your first post, you said “The arrow of the ASI points directly at your desired value“.

So my troubleshooting would be exactly as stated in my last post.

Now, you’re saying the ASI was indicating a slower than expected speed. Which would start to suggest a performance/engine issue.

which is is?

Share on other sites

As of this point, OME has been relieved of his fill-in CASA job, writing out exam questions.

It has been suggested he move to a position in corporate financial management, where he can make a career out of obfuscation.

I'm afraid I was struggling to figure out what could have been going wrong, but I did suggest it was due to lower engine power output than expected.

• 1
Share on other sites

We can be thankful the winds are light for the landing. Looking forward to the next enlightenment.

Share on other sites

Okay, here's an easy one:

An aerial wildlife survey, undertaken in nil wind conditions, and which located a bear and several colonies of birds, where the pilot flew due east for 30 minutes, north for an hour, then south for an hour, returning to base.

What colour was the bear?

• 1
• 1

White

• 1

Share on other sites
12 hours ago, M61A1 said:

You stated that the ASI was reading exactly where it should

I said that the ASI was exactly where it should be when "You take off and set for cruise". I didn't say anything about the ASI reading 30 minutes later. All I said was that 30 minutes into the flight you notice that you were running late.

10 hours ago, Ryanm said:

you’re saying the ASI was indicating a slower than expected speed. Which would start to suggest a performance/engine issue.

Why didn't you explore that idea further. You were on the right track.

People mentioned a blocked pitot tube, which can happen for a variety of reasons. If the blockage occurs on the ground, you will notice that your air speed indicator continues to read zero even as you begin to accelerate for takeoff.

An in-flight blockage likely won’t be noticed as quickly, however since the air speed indicator measures the difference between static and dynamic pressure, a blocked pitot will skew your dynamic pressure reading. When you ascend or descend, the static pressure will change while the dynamic pressure will remain artificially constant. Therefore, regardless of true airspeed, indicated airspeed will decrease as you descend and increase as you ascend.

In this exercise, the aircraft remained flying straight and level at the desired cruise altitude, so the the static pressure would not change. The ASI is reading lower than expected, so you could discount a blocked pitot tube.

The effect of loss of efficiency due to a failed magneto might only be small, but over time that can amount to a longer time to cover a set distance. As an example, taken from the POH of a C-152,  75% power at 4000' should yield 103 kts, while 65% power will yield 97 kts (still wind conditions). It's a case of pennies make pounds, or for you young bloods, cents make dollars.

The exercise was aimed at testing both knowledge of aircraft systems, and the ability to develop a systematic procedure to diagnose a problem. Nobody produced any sort of procedure, just jumped on one thing.

I will take a it on one thing - airmanship. Thirty minutes into a flight and a fault is discovered, yet I am prepared to press on. A wise pilot would have turn back to Base, if only to ensure that s/he could get to s/his car to drive home.

Share on other sites

Whether you can safely complete the trip.. Could easily be you have a heavier plane than you thought which will gradually correct the figures as you burn fuel.  I got called away Fits with OME's 3 hours difference  to this  one This is at least a day old.  but still valid. Perhaps you would expand on EFFECT of the risk element and how you justify not continuing. just because there is "some" fault. Aeroplanes operate frequently with faults  and have permissible unserviceability lists  or minimum equipment lists used at the PIC's discretion. depending on the circumstances prevailing. Nev

• 1
Share on other sites

In fairness, OME, you said:

"RPM set to 75% cruise power."

and

"The arrow of the ASI points directly at your desired value."

I assumed from your scenario that was also the case 30minutes later, since I can't imagine any pilot would set and forget the throttle. Would they?

Clearly, if ground speed is less than expected, then either the air speed is down, or there is some wind situation impacting the ground speed.

Your scenario implied (to me) that the air speed was not down.

Perhaps that was why the responses were not as you had expected?

• 3
• 1
Share on other sites
37 minutes ago, IBob said:

I assumed from your scenario that was also the case 30minutes later, since I can't imagine any pilot would set and forget the throttle. Would they?

Did you question your assumption? The thing to consider is that after a period, the pilot first notices that something is out of place. It's a minor thing - running behind ETA. Shouldn't the pilot ask, "Why?" and then diagnostic process starts from there?

Let's look at the circumstances of the flight. Lovely day for flying; little or no effort required in pushing the aircraft around to counter the effects of wind. A small drop in RPM would hardly be noticeable from engine noise. Honestly, do pilots spend the whole of a recreational flight intensely scanning the instruments? Isn't the rule, keep your eyes out of the aircraft? Unless the pilot is aware, or has been warned, of the possibility of the throttle creeping closed, why would the pilot touch it after setting and securing it?

46 minutes ago, facthunter said:

Perhaps you would expand on EFFECT of the risk element and how you justify not continuing

That's a really good topic for another thread, but discussing it at the moment here would distract from the original reason for the exercise.

I do agree that aircraft can be operated with permissible unserviceabilities, but in this case which is a superior result - continue to destination away from home and possibly being stuck there with a U/S aircraft, or turn back to base and get to sleep in your own bed and deal with the problem at your leisure? Sort of a reverse case of "get-there-itis"

What I have tried to make people think about in this exercise is formulating a systematic process to diagnosing a fault in an aircraft system. That process depends on knowledge of aircraft systems. I'm sorry to say that everyone has not met the required standard.

Share on other sites

No, I didn't. Partly because of the way the scenario was phrased:

"The arrow of the ASI points directly at your desired value."  When clearly, according to you, it does not.

And partly because I am very aware of the RPM of the engine, and find it hard to imagine a pilot would just lock the throttle and take no further notice.

Maybe it depends on what sort of flying one does...........

Share on other sites
5 hours ago, Thruster88 said:

White

And Thruster88 gets the ceeegar..........)

Share on other sites
27 minutes ago, old man emu said:

I'm sorry to say that everyone has not met the required standard.

Being in good company always makes me feel less of a failure. 🙂

• 1
Share on other sites
1 hour ago, rgmwa said:

Being in good company always makes me feel less of a failure. 🙂

Don't feel a failure. The failure occurred when the question was written. Not when it was answered.

I have had an instructor that used to asked such vague and broad questions. it seemed as that way he could always be right and you would always be wrong, because the answer could be whatever he wanted it to be.

Share on other sites
Quote

What I have tried to make people think about in this exercise is formulating a systematic process to diagnosing a fault in an aircraft system

So, essentially, what you're saying is - if your kitbuilt aircraft doesn't come with a POH, you should write one up, and include the important emergency operational checklist?

Share on other sites

When setting this sort of conundrum I would expect to be able to rely on the information given. It is easy to change the info and not tell us, but there is no way for us to work out what is happening. From the info given I assumed that rpm was correct as it was stated. I still reckon that given the original info and accepting it all as correct, one thing that could cause a drop of GS would be a land breeze coming of the ocean. Remember the destinatiion was a beachside and land breezes are common in the PM.

Try another one OME, this has certainly got us all thinking.

• 2
• 1
Share on other sites

I fly Recreational not GA so I don’t care. The world as I know came to an end when Shell stopped printing road maps.

Ken

• 1
• 1
Share on other sites
2 hours ago, IBob said:

In fairness, OME, you said:

"RPM set to 75% cruise power."

and

"The arrow of the ASI points directly at your desired value."

I assumed from your scenario that was also the case 30minutes later, since I can't imagine any pilot would set and forget the throttle. Would they?

Clearly, if ground speed is less than expected, then either the air speed is down, or there is some wind situation impacting the ground speed.

Your scenario implied (to me) that the air speed was not down.

Perhaps that was why the responses were not as you had expected?

I have experienced this exact situation when cross checking another altimeter in flight. I have two instruments that read airspeed and disconnected the static off one to connect to the altimeter being tested. It was noted immediately that the ASI with cockpit static pressure was overreading significantly compare to the one with a proper static port and the GPS.... Virtually this exact scenario.

• 1
Share on other sites

Onetrack. Is it legal to fly a kitbuilt or any other type of aircraft without a POH or some other type if instruction manual. I would consider it extremely foolish to fly without reading the POH. I have done it, but after a great deal of questioning of the owner about systems and flying characteristics. I have written 2 POHs' for the planes I have built. Even flying a similar plane it would be sensible to read the POH, as I discovered with a C172 demo plane. I flew it with the agents pilot and on final I did what I normally did in a C172, pushed down on the flap lever for 10 deg of flap and found that I had to push the nose down to maintain speed. The new planes flap lever had to be pushed down and then up when the required flap had been set.

• 1
Share on other sites
24 minutes ago, onetrack said:

So, essentially, what you're saying is - if your kitbuilt aircraft doesn't come with a POH, you should write one up, and include the important emergency operational checklist?

No. What I was after was for people to go back to basics and work out, from their knowledge of AIRCRAFT SYSTEMS, what could have been the cause.

You can't tell me that when you go flying and conditions are really nice that you don't relax and watch the world go by? Or are all flights spent in fear of structural, mechanical failure or dramatic changes in "weather" that you are required to spend the whole flight assiduously gazing at your instrument panel?

21 minutes ago, Yenn said:

Try another one OME, this has certainly got us all thinking.

Thanks Yenn. That was the object of the exercise.

Nobody twigged to the fact that at top of climb everything was set up by the pilot for cruising, so at that time all the needles were pointing at what was wanted. It wasn't until 30 minutes later that the pilot noticed an anomaly - behind ETA - which would have started a search for a reason. That's when the reading of the ASI would have been noticed and, to quote Professor Julius Sumner Miller, ask "Why is it so?"

I'm amazed that no one has commented about the degree of RPM loss from the set RPM would be caused by a magneto failing. When I first thought of this, I figured that the reduction in RPM would have been a bit over 50, but not over 100. That would have been the same, say as going from a desired 75% power to maybe 70%, with a consequent loss of thrust leading to a reduced airspeed and therefore ground speed and time.

Share on other sites

My flights are spent with one hand on the throttle all the time. The cockpit is too small for it to be anywhere else.

On Saturday I was stooging along and all of a sudden saw that my Tiny tach was indicating over 5000rpm.

That is the only tach I use and it was new only a few hours ago. The Jab engine will not run to 5000rpm.

I know what happened, do you? I will fix it before next flight.