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Cessna 340 - lookin' for advice...

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Cessna 340 - lookin' for advice...

Old 14th May 2008, 15:00
  #41 (permalink)  
 
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I believe this is the best method to pilot a 340A on takeoff. Advantages are;

* You have adequate runway for a rejected takeoff.
...
* You immediately accelerate to Vyse and then select gear up as THE decision making moment to commit to the climbout, regardless of engine failure, and treat an engine failure as an in flight emergency.
Interesting -- and very similar to what we do in the Twin Comanche (Vyse 90 kt). How much runway do you need accelerate to 100 kt and then stop? Wouldn't the published accelerate-stop distance be for 91 kt?
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Old 15th May 2008, 00:04
  #42 (permalink)  
 
Join Date: May 2008
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340A

The takeoff roll, depending on GTOW, DA and temperature is about 1,400 ft. The accelerate stop is about 2,700 ft., according to the book. I have landed and departed in runways as short as 2,700 ft, never again. I admit to being spoiled at my local airport SUS to have a very well maintained 7,500 ft runway that I now accept as “normal”.
Lifeisgood is offline  
Old 15th May 2008, 09:43
  #43 (permalink)  
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A few final thoughts

Hi Bookworm.Yep, I take your point, about rejecting takeoffs in twins for minor engine problems. I'm coming to this issue with a fair amount of experience in short field ops in singles, and also maybe 400 hours in piston helicopters, but not a great deal of M/E time, of which I probably only have 150 hours or so. I did train comprehensively for my M/E rating with a very experienced instructor, have read a lot on the subject, and found my flight test to be quite demanding and comprehensive. I appreciate you taking the time to discuss the issue rationally, rather than rushing into ad hominem attacks cos I’m supposedly asking “stupid questions”. I originally decided to stay out of this issue once the shouting started, but perhaps one more post would be pertinent, particularly since you've added some thoughts that are of value. Thank you.

In both the single STOL situation, and taking off in a piston helictoper - or for that matter flying over, say, forests, in either, one is frequently in situations where an engine failure at an inopportune moment could cost you very dearly indeed. The scenario you paint, in terms of a STOL takeoff in a twin, doesn't seem entirely dissimilar, to me, than that of taking off in a single – EXCEPT for the VMC issue. Once you're committed to a STOL takeoff, or a helicopter takeoff on the not unheard of occasions where you can’t stay within the H/V curve, if you have a 'non-immediate failure' engine burp/twitch/whatever, then you're potentially in a world of hurt. In a single, you have a millisecond to decide between the rock and the hard place, and you're certainly not operating on a balanced field. So the real question, it seems to me, is back where we started – does retarding the power on the good engine prevent a VMC roll or not? It would appear the consensus on the answer is no – a VMC roll, or serious loss of directional control, is inevitable, EVEN IF you immediately retarded the power on the good engine. Well, to those who’ve adoped that position, fair enough. I hear ya. I won’t take off below VMC. I was never intending to. It would have been nice to understand exactly why not, rather than take it at face value, and there are one or two answers here that make a strong case in terms of explaining WHY you can’t retard power on the good engine in time. I’ll think more about this issue, and keep picking the brains of those more experience than me. But, just so you know, I ran this by a 30-years-flying M/E guy yesterday, and guess what? He reckons directional control is quite achieveable, provided you cut power on the good engine immediately. I guess that makes him stupid too? Maybe we’re ALL stupid down here in New Zealand. Or maybe the coriolis effect changes the aerodynamics J

Most pilots spend a great deal of time planning & training for engine failures, either partial or total - but my understanding is that statistically speaking, avoidable engine failures - ie not produced by fuel starvation or mismanagement - are extremely rare, which is why some people choose to accept the risks involved in single engine STOL ops, or piston helis. (And, for the record, I HAVE in fact had 2 engine failures, in 12 years of flying, one arguably my fault to some degree, one unavoidable, so I don't believe they're impossible. Just very uncommon.) In terms of this particular issue, I'd have to stand my my initial comment, whatever the angry poster guy thinks of my training, approach to risk, suitability to hold an M/E rating, or whatever. I STILL "can't really see the difference between flying off a short field in a single, and accepting that if the engine fails in the first 5-10 seconds after airborne you're in deep trouble, and doing the same in a twin", EXCEPT insofar as it is affected by VMC. (And no, angry poster guy, save your aggression & go & kick your dog or something - you've made it pretty clear you're here to preach not to listen).

I'm sorry if I appear stupid to the more experienced people here, but I'm used to the concept of relying on my engine, and to minimising as much as possible but never eliminating the risk entirely, if that engine decides to quit. This is why I've been trying to work out if a VMC roll is inevitable if you rotate at a slow speed, by which I don't mean, necessarily, BELOW VMC - but possibly 5 knots or so below the published rotation speed. Or 10 knots, even. My personal opinion is that factory charts are helpful guides but NOT the singular, authoritative and definitive performance indicators that we would like them to be, so I like to augment the published data with actual date, by checking the actual aircraft's real world performance against the charts. The aircraft may or may not be in "as new" condition, for instance, it may be operating off fields that are neither flat concrete nor "firm sod - add 7%" - which are the only two runway surfaces options in my 340’s performance charts - and the published rotation speed of 91 knots may be slightly on the high side if you're operating at very light weights. For instance.

I was considering the possibility that the risk might be basically double that of a single - ie that if you have an engine failure at slow speed, you may accept that you are going down, but straight, level, and in control, but the chances of an engine issue are, of course, doubled. If it was the case that a well handled engine failure, at worst, involved a controlled level crash/overrun, well, that's one thing, and worth considering in terms of operating out of the field I’m considering, which, by the way, is 2400 feet long, so it’s short for a 340, but possibly not exactly a postage stamp. If a VMC roll is inevitable, likely, or even possible, then of course that's a whole different ball of wax. This is what I've been trying to find out. In between the invective, there's been a lot of worthwhile discussion, and I thank you guys for that.

Angry poster guy quoted me on a different thread as "(postulating) that as directional control may not be available on the ground after an engine failure, he sees no reason why one should not take off below Vmca." This illustrates one of the limitations of written communication in general, and particularly where goodwill & respect aren't present. What I actually said was

"What I don't understand is why, if you were hair-triggered to cut power on the live engine, as close as humanly possible, as well as applying immediate opposite rudder (aileron?), then wouldn't the removal of the live power remove the roll/yaw immediately, and thus turn it into a common-or-garden engine failure situation? "

The key being "I don't understand why". I didn't say that I DO understand why, I said I DON'T understand why. I also didn't say that I take off below VMC (I don't) nor that I plan to (I don't.) I was trying to answer a question that I confuses me, and I’m sorry to say I still don't really understand. My original question was “Anyone know what the shortest possible/reasonable/safe takeoff would be for a 340 operating at very low weights?”. One of the posters on the thread has kindly PM’d me with a concise and reasonable answer to that particular question. If I somehow conveyed an intention to fly unsafely, well, it was unintentional. Again, written language, without the non-verbal cues & feedback of a conversation, is easily misinterpreted, which is why I think it’s important to read these anonymous posts with goodwill and an open mind. But having answered that question, I’m STILL not sure why you can’t cut the power on the good engine, and end the assymetric thrust issue, turning your engine failure into a double engine failure, but at least going in under control, and right way up.

Yes, I've read this thread several times, and I understand the definition of VMC ".........critical engine suddenly inoperative with the working engine at maximum rated T/O power in T/O configuration (=gear up)", and I understand why that leads to roll and yaw that cannot be overcome. But I still don't understand why that roll and yaw could not be stopped when you cut power on the operative engine, unless rote adherence to procedure qualifies as understanding. In my experience, I would estimate that power reduction on the good engine would take a about a second or so to take effect, not the four or five seconds referred to by one of the contributors to this thread. (And, of course, the failed engine is likely to spin down at a similar rate, rather than stop dead.) I would have thought that the immediate power reduction, COMBINED WITH opposite rudder/aileron (which will, of course, be effective to some degree, just not enough to overcome the yaw & roll moment of a "working engine at maximum rated T/O power" might, in fact, be enough to prevent the yaw/roll combination that could induce a VMC roll.

I realise that what I "would have thought" isn't enough basis to go and fly the machine in a manner different to that written down in the manual. I'm not suggesting I'm going to. I'm trying to educate myself here. I am mystified by the concept that "conflict hones the skills and allows everyone to reasses their values and opinions". Personally, I find that reasonable discussion is a more effective route to reassessing "values & opinions" than trading barbs about how stupid someone is. And, of course, training in the aircraft - and in the simulator, where some scenarios are too dangerous to enact in the real aircraft. I am indeed heading off to Flight Safety, and will continue to explore the questions raised in this thread, in a safe manner.

It's a shame that these threads, which can be so educational, so easily degenerate into people talking to each other in a manner that I doubt very much they would do so mano o mano, so to speak, unless they were drunk. Actually, maybe that explains it!!! Maybe some of these guys are drunk! I see now

So. I have a great deal to think about. As it happens, VMC on my aircraft is about the same as the stall speed, due to the vortex generators, so it may be that I don't need to even think about this issue. And according to the published charts, the field, at 2500 feet, is long enough for operations at light weights, provided the (grass) surface isn't too soft, and with the caveat that “balanced field” ops aren’t possible. But we'll see. In terms of real world actions, from here on in, I am indeed going to attend a Flight Safety course, where I hope it may be possible to accurately simulate a light weight 340 with VGs operating off a grass field. I doubt if I'll posting the video for Lifeisgood & his friends to scoff at over their beers, but yes, videoing it for later review is a very good idea, and I will do that. I'll then, before I even think of flying out of this particular field, do plenty of flying off a l-o-n-g grass runway, to get completely current on the performance of the machine at light weights. As it happens, whatever misunderstandings angry poster guy et al may have come to, I quite agree about the need for safety in all aircraft operations. And if I have somehow suggested things that seem to belittle airline pilots and airline operations, well, I apologise for that. It wasn’t my intention. I think the safety record of the airlines is outstanding, and that private ops should indeed aim to be as safe as possible. It is my **personal opinion** that many PPLs do have more freedoms and privileges than many airline pilots, in that we are the sole decision-makers, within the bounds of legal regulations, when it comes to go/no-go decisions, routes flown, airstrips/airports used, weights flown at, fuel carried, (often) maintenance carried out/deferred and so on. I’m sure that’s one of the many reasons that private flying has a worse safety record than the airlines, not that I have ever said I think that’s a good thing. Note, however, that I didn’t say that PPLs have more responsibility. Obviously flying with 50, 100, or 300 people’s lives in your hands involved a great deal more responsibility than when you’re just dealing with your own life, and perhaps those of your family and friends.

So thank you, y'all, even angry poster guy, for your responses. There is indeed a lot of wheat amongst the chaff. You've all given me lots to think about, and no, I won't be racing off to kill myself in my 340.
lostpianoplayer is offline  
Old 15th May 2008, 11:55
  #44 (permalink)  
 
Join Date: May 2008
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340A

I can tell you that I have attempted Vmc demos in my 340A last year at a safe altitude, 10,000 AGL, just to see how it would behave. I trained in the venerable Beach Duchess, and as benign as she looks, she can be a handful if you push the power in too quickly in the Vmc recovery maneuver. I once had a Duchess with wings almost vertical in less time than you can say “OH SHIT”. I quickly learned you ease the power in, don’t cram it. Anyway back to the 340A in a Vmc demo.

I could not get the 340A to roll.

There was adequate rudder and aileron control right down to a stall buffet at about 78 kts! I was configured at GTOW less 800 lbs, which should have raised Vmc, I was also at 10,000 Ft AGL, which I do not think affected the Vmc Demo because the turbochargers compensated for DA with the engines and the airspeed indicator and wings simply compensated for the DA via indicated airspeed. At this point I conceded that the Vmc was below stall and did not wish to become a test pilot doing spin recoveries in my favorite twin. It would be interesting to know if a Simulator can be configured at a lower weight than MGTOW and if the flight characteristics will be modified to that minute detail, I doubt it.

I can also tell you the improvements in performance in a 340A when lightly loaded are spectacular. In my 340A with just myself and half fuel it’s a rocket on takeoff. I usually see in excess of 2,200 fpm on takeoff.

So the short answer to; can it be done; yea, probably.

Should you do it; only by yourself.

But the better question is; why would you want to?

In retrospect, the negative comments you received were doubtless due to a few pilots expressing their dismay, me included in that group, of you suggesting a very high risk maneuver. You are responsible for your decisions, and you certainly seem to be well trained in all pertinent aspects of this flight envelope. To attempt a takeoff below Vmc is contrary to the POH and countless other rules, regs and best practices. You asked for advice on an unsafe, foolish maneuver and received chided demonstrative responses you did not appreciate. Go figure. The entire aviation industry and culture is invested in safety, training and sound piloting. The 340A is not a short field aircraft, to attempt to use it as such is placing it a flight envelope it was not designed for and is not a sound decision.

That having been said, Best Whishes.


Lifeisgood is offline  
Old 15th May 2008, 15:45
  #45 (permalink)  
 
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I can tell you that I have attempted Vmc demos in my 340A last year at a safe altitude, 10,000 AGL, just to see how it would behave.
How the airplane behaves at 10,000' and at sea level, particularly on takeoff, will be two very different things, particularly as effective Vmc (the actual speed, rather than the certification number) decreases with altitude.

In retrospect, the negative comments you received were doubtless due to a few pilots expressing their dismay, me included in that group, of you suggesting a very high risk maneuver. You are responsible for your decisions, and you certainly seem to be well trained in all pertinent aspects of this flight envelope. To attempt a takeoff below Vmc is contrary to the POH and countless other rules, regs and best practices. You asked for advice on an unsafe, foolish maneuver and received chided demonstrative responses you did not appreciate. Go figure. The entire aviation industry and culture is invested in safety, training and sound piloting. The 340A is not a short field aircraft, to attempt to use it as such is placing it a flight envelope it was not designed for and is not a sound decision.
Well said, and that said, sometimes one simply cannot give a drowning man a glass of water.
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Old 15th May 2008, 16:35
  #46 (permalink)  
 
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Lostpianoplayer.

From my point of view the real issue in this thread should not be how to squeeze a C340 into a role it was not designed for but rather what aircraft meets your operatonal requirements of

1) be multiengine

2) operate out of a 1600 ft strip at 2500 MSL with 1 or 2 pasengers

3) can be safely operated by a relatively low multi time pilot

I would suggest you should look at a Roberston STOL turbo C337 skymaster.
This aircraft would in my experience meet all your requirements and of course does not have the VMC issue.
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Old 15th May 2008, 16:54
  #47 (permalink)  
 
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In both the single STOL situation, and taking off in a piston helictoper - or for that matter flying over, say, forests, in either, one is frequently in situations where an engine failure at an inopportune moment could cost you very dearly indeed. The scenario you paint, in terms of a STOL takeoff in a twin, doesn't seem entirely dissimilar, to me, than that of taking off in a single – EXCEPT for the VMC issue.
I think the difference is that in the single, in the case of a non-catastrophic engine problem, you leave the throttle fully forward and fly the aeroplane. You probably look for somewhere, or a selection of sites, to put it down straight ahead. If you're still climbing, bonus, the engine failure wasn't as bad as it might have been and you keep climbing. If the engine loses power completely, you're going to glide to a landing site as best you can.

In the twin, you really have the decision to make, right then and there, before you have much chance to properly assess the problem. If you don't make the decision to close both throttles, and the engine goes on to wind down to complete failure, you're flying yourself into a loss of control.

By contrast, in a similar situation in the climbout at Vyse or above, you have a little more time to assess the situation, and, just as in the single, can afford to leave the problem engine running to see what happens. Of course you can't wait forever, but you're not going to shut it down at the first cough at lowish level if there's a very good chance that it will still be producing useful power.
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Old 15th May 2008, 16:54
  #48 (permalink)  
 
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Lostpianoplayer.

From my point of view the real issue in this thread should not be how to squeeze a C340 into a role it was not designed for but rather what aircraft meets your operatonal requirements of

1) be multiengine

2) operate out of a 1600 ft strip at 2500 MSL with 1 or 2 pasengers

3) can be safely operated by a relatively low multi time pilot

I would suggest you should look at a Roberston STOL turbo C337 skymaster.
This aircraft would in my experience meet all your requirements and of course does not have the VMC issue.
Big Pistons Forever is offline  
Old 16th May 2008, 04:50
  #49 (permalink)  
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Lostpianoplayer - over & out

Bookworm, re getting airborne w a questionable engine, I guess my point was just that the same thing could happen in STOL operations, where you have to make a quick decision about taking it airborne with a POSSIBLY malfunctioning engine, but as you're rapidly accelerating on a short field, a true short field, you have a millsecond to cut power and brake, and possibly still go through the fence at the end. OR get airborne with a problem that could get a lot worse, very fast. I'm not saying this is a good thing, just that the phemenonon exists, and people nonetheless operate STOL in singles. I can see how it could be more of an issue in a twin, at least in part cos you have more coumns & rows in the decision matrix.

Re the choice of machine, BPF, thank you, and yes, you're right - a turbo Skymaster WOULD be a good strip machine. But the 340 fills other needs/desires I have, very well, and I have other aircraft to use for the short strip role. I've always been prepared to base the 340 at the local airport (runway 5000 +), but have been trying to get a thorough understanding of WHY, not really WHETHER I shouldn't base it at my own (shared) private airstrip, which, at 2500 feet long, is not a short strip for many machines. (The 1600 feet referred to in my first post, incidentally, was published takeoff for a 340 at full gross, not the length of the airstrip.) My instructor & flying mentor, with whom I have been flying for 12 years, and I didn't have a clear consensus on the VMC issue, and I decided to ask here to advance our own knowledge. We'll be discussing this issue over dinner some time, and he was a little hazy on some aspects of VMC himself, despite having telephone number hours in all sorts of different aircraft. I'm not, and was never, looking for ammo with which to veto his advice - just to find out more, and UNDERSTAND the situation. Especially as I was told when training, that the first thing you do to stop a VMC roll was to cut power - which would obviously improve the situation to some degree. The question is how much. It sounds like some are of the opinion that a VMC roll, once started, is unstoppable, and I hear ya. I'm not sure I agree (yet), but will take this issue further. I'll certainly do my best to see if the good folks at Flight Safety will demonstrate it all to me, and have no doubt that they'll be happy to do so. So yeah, thanks for the suggestion of a more suitable machine, but that's not really what it's about. It'll be a long runway for me & the 340, for the forseeable future.


Re dismay, drowning men, glasses of water, etc, well, whatever. A lot of what I wrote was taken out of context or misunderstood, interpreted in its worst light, or whatever. Partly my fault for pasting over a question from a different forum, and mixing up practical questions like question (1) (what is the shortest reasonable/possible/SAFE takeoff, ***which by definition means ABOVE VMC***) with theoretical questions like question (2). IF (IF!!!!) you took off slow, and had a failure, then why wouldn't retarding the assymetric power stop the roll/yaw.) I'll make sure that if I venture over here again I'll keep the questions very simple, don't confuse the thread with multiple questions, and make sure that I make the caveats, context & exceptions clear. BTW, Guppy, the whole POINT of "taken out of context" is that one IS, indeed, "quoted". But quoted without the context, as you'll see on the news every day, can totally distort the meaning. One aspect of this thread which I find very interesting, is that written language is so limited, and can be easily misunderstood, either in tone or in substance. You could consider my two first questions, for instance, and pretend just for a moment that the person who wrote it has no intention of taking off subVMC, or anywhere near it, unless he can be satisfied that a VMC roll can be stopped by retarding the other one. And he doesn't understand, at that stage, why it CAN'T be stopped by retarding the power on the good engine. Maybe the questions might be seen in a different light then, rather than their author 'planning to do something really dangerous', 'condemning safety' and so on. (Only do this if you find linguistics enlightening, and don't actually get off on having unncessary arguments, which of course, some people do!) I don't care about my aviation reputation amongst a few anonymous computer users - I just think the aggressive tone obscures what is often very helpful substantive information and opinions. And I find it fascinating how easily information can be & is misinterpreted or twisted. It demonstrates, for instance, the limitations of flight manuals, per se, without clarification & further training - or the reason that angry emails to, say, your employer tend to backfire. Effective communication relies heavily on context, clarification & feedback. Anyway. I think we're well beyond the 80/20 on this thread, and this time I really am gonna sign off, unless something takes an interesting turn. Feel free to PM me if you want to take things further - and thanks, really, to the people who have PM'd me about the questions I asked at the beginning, with what has been a helpful boatload of information, and also to everyone who's contributed to my understanding of this issue. Particularly those who provided real world 340 data even if they may have misunderstood the point of my questions, and my intentions. Fly safe, and cheers.

Last edited by lostpianoplayer; 16th May 2008 at 04:53. Reason: typo
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Old 16th May 2008, 12:30
  #50 (permalink)  
 
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Vmc

Regarding “if Vmc speed reduces as altitude increases”. I pondered this question for a quite a while, discussed it with knowledgeable CFI’s and commercial pilots and would appreciate input from others.

Is this true in a turbocharged aircraft or for that matter any aircraft that is flat rated to a given altitude? I believe this generally held belief is due to DA decreasing with altitude and consequently less air molecules available for combustion, and therefore less power produced, and therefore less torque, and therefore a lower Vmc speed in non-turbocharged aircraft.

Regarding the wings; Because DA reduces with altitude there are less molecules of air hitting the front of the Pitot tube, so therefore we are trained to understand and use indicated air speed, IAS. Or to paraphrase, IAS is always used by pilots because IAS automatically compensates for DA. Therefore as pilots we understand that IAS is not TAS but to use IAS for all maneuvering decisions and understanding that true airspeed is higher due to effects caused by DA.


As per WIKI.
“Unless an aircraft is at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) and no wind, the IAS bears little relation to how fast an aircraft is moving in reference to the ground; however, because the air pressure and density affect IAS/CAS and an aircraft's flight characteristics in exactly the same way, IAS and CAS are extremely useful for controlling an aircraft, and the critical V speeds are usually given as IAS. In aneroid instruments the indicated airspeed drops-off with increasing altitude as air pressure decreases, and this leads to an apparent falling-off of airspeed at higher altitudes.”

So the question is;

Does the IAS Vmc speed reduce in an aircraft that has flat rated engines as DA increases?

Opinions appreciated.
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Old 16th May 2008, 16:58
  #51 (permalink)  
 
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Lostpianoplayer

I did C340 recurrent training in SIMCOM's sim. I found the sim very accurately replicated the flying qualities of the real airplane. It was hands down the best training I have ever done for light aircraft. I got to see every possible ugly takeoff scenario you could think off. This is what led to the personal decison to delay rotation to 100 kts as I found I could fly away every time. I hope you enjoy your time at Flight Safety. One note of caution. My buddy the high time owner of the C340 I used to fly was rather unhappy with the instructor he had at FS last fall. She was very inexperienced and would not deviate from the check box training form she was given. I would recommned you discuss what you want to do in your training with Fight Safety management before you arrive in order to ensure you get an instructor who will give you what you want. You should also compare SIMCOM's program before making up your mind.
Big Pistons Forever is offline  
Old 16th May 2008, 18:34
  #52 (permalink)  
 
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Is this true in a turbocharged aircraft or for that matter any aircraft that is flat rated to a given altitude?
Yes, it's true, but the decrease in the effective minimum single engine control speed (which isn't the same as Vmc...Vmc is a certification number) isn't as large with a turbocharged, supercharged, turbosupercharged, or turbocompound engine.

The engine can only output so much, and regardless of whether the engine output remains the same (a turbo normalized engine, for example), the propeller efficiency decreases with altitude due to a decrease in air density. Less thrust, less assymetrical thrust, lower real minimum single engine speed required to achieve the same result.

Remember that Vmc is a function of assymetrical thrust, which is an indirect effect of engine performance. The engine still has to turn the propeller, which must act on the atmosphere to produce that thrust. Decrease the thrust (a function strictly of blade angle, air density, and RPM), and you decrease any potential for thrust assymetry, and therefore the value of "Vmc" decreases. Just the same as if you reduced power on the good engine.

Vmc is a solitary number, and in most cases, it doesn't go up or down appreciably. Some AFM's publish variable numbers, but it's generally given as just a red radial reference line on the airspeed indicator. That aircraft control is actually lost at this point is merely a coincidence. Numerous factors can decrease the actual speed where directional control isn't possible, and various factors can increase the number...all while Vmc remains the same. Actual performance isn't the same as Vmc. Vmc is a paper number, not what's going on in the airplane.

We generically refer to what's really going on in the airplane as Vmc to make it simple, but this also tends to confuse the issue sometimes when talking about the science behind it.

I believe this generally held belief is due to DA decreasing with altitude and consequently less air molecules available for combustion, and therefore less power produced, and therefore less torque, and therefore a lower Vmc speed in non-turbocharged aircraft.
Density altitude increases with altitude, and with an increase in air temperature. It does not decrease with an increase in altitude except in the case of a strong temperature inversion, and then only for brief periods.

Regarding the wings; Because DA reduces with altitude there are less molecules of air hitting the front of the Pitot tube, so therefore we are trained to understand and use indicated air speed, IAS. Or to paraphrase, IAS is always used by pilots because IAS automatically compensates for DA. Therefore as pilots we understand that IAS is not TAS but to use IAS for all maneuvering decisions and understanding that true airspeed is higher due to effects caused by DA.
The wing is irrelevant to the question, because Vmc is a function of assymetrical thrust and control availability (chiefly rudder). Center of gravity makes a difference, but the reason the minimum control speed decreases with altitude (again, DA, or density altitude does not decrease with an increase in altitude) is a decrease in assymetrical thrust. This is more pronounced in a normally aspirated engine, but still exists in a turbocharged engine for the reasons previously given.

Does the IAS Vmc speed reduce in an aircraft that has flat rated engines as DA increases?
Again, Vmc may never change, because it's a single number, and is painted on your airspeed indicator. The actual number at which directional control can no longer be maintained (the effective Vmc, if you will), does decrease...even if the engine is normalized to maintain the same power output as altitude decreases. The engine still has to drive the propeller. The engine doesn't produce any thrust of it's own accord (save for a few circumstances where exhuast gas does produce a very slight thrust quotient). It's the propeller doin the work moving the air, and it's propeller efficiency that decreases with altitude.

Additionally, while one flies the same airspeeds with an increase in density altitude, as one climbs the air density and aircraft reaction does not remain in proportion. This isn't greatly noticable at lower altitudes, but it is the higher you climb.
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Old 16th May 2008, 18:57
  #53 (permalink)  
 
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The engine can only output so much, and regardless of whether the engine output remains the same (a turbo normalized engine, for example), the propeller efficiency decreases with altitude due to a decrease in air density.
I don't doubt that propeller efficiency plays a part. Also worth remembering that power is thrust x TAS. For the same IAS, TAS is higher at higher DA, therefore thrust is less for an engine flat-rated to a power.
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Old 20th May 2008, 11:54
  #54 (permalink)  
 
Join Date: May 2008
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Vmc

I’ve read up on propeller efficiency loss at altitude, there seems a lack of readily available information on this. As a matter of curiosity I would like know what percentage of loss occurs from sea level to FL200 for a constant speed propeller. In my previous article I mentioned wings being affected by DA, I should have said airfoils, which includes the rudder. Meaning that all airfoils are affected by less wind crossing their surfaces and eventually control is lost.

This is all a moot point, because we are talking about controllability at the edges of the envelope. To know which force will cause the Vmc roll is hardly necessary; is it the lack of rudder, the roll from loss of aileron control, the torque of the engine or a concert of forces working together to kill the pilot and passengers.

The best practice, and that which we have all been taught, is to avoid these death traps with prudence and good piloting skills.

This has been an enjoyable but somber rant.

Adieu
Lifeisgood is offline  
Old 20th May 2008, 17:43
  #55 (permalink)  
 
Join Date: Jan 2004
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Last week a well maintained corporate C421A went off the runway at my home airport. Loss of control was caused by a broken scissor bolt which allowed the right main wheel to rotate 90 deg. It is in the shop waiting for a whole new gear set as all the legs failed NDT inspections. The pilot felt a contributing factor my have been its regular operation into a sea level 2400 ft paved strip. This invariably required firm touchdowns and immediate very heavy breaking. As twin Cessna's have a well known reputation for weak landing gears perhaps this is another reason you do not really want to be forcing them into short strips.

WRT the rather arcane discussion on prop efficency and VMC I second lifeisgood's sentiments. It is all about not going anywhere near the edge of the cliff. In any case my expereince has been that the all of the scary moments in my approx 2000hrs of instructing invariably involved instructing low houred PPL/CPL's for the ME rating (a total of maybe 60 hrs of the above 2000hrs). it sure gave me a healthy respect for the dangers of one engine operations in the low speed regimes
Big Pistons Forever is offline  
Old 20th May 2008, 18:08
  #56 (permalink)  
 
Join Date: Oct 2005
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On the subject of instructing the multi engine student, I have always practiced blocking the rudder pedal during single engine work such that the student does not have full rudder throw available. In the even the student doesn't react quickly enough or properly, the remaining rudder throw is mine to maintain control and recover.
SNS3Guppy is offline  
Old 30th Jul 2020, 05:52
  #57 (permalink)  
 
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Hi folks,
I'm resurrecting this old dog for a specific question regarding the 340. I came across a decent one recently, which is collecting dust and the owners don't know what to do about it.
The pilot told me that it flies great, however it has a persistent problem with the fuel pumps transferring fuel from the aux to the main tanks. The pumps short all the time, thereby rendering the aux tanks unusable and shortening the range considerably.
Is it a know issue of this type?
Many thanks in advance.
DirtyProp is offline  
Old 30th Jul 2020, 11:06
  #58 (permalink)  
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Two things come to mind with this situation. It's been 35 years since I've flown 340/310, so I'm going from old memory, but; the aux tanks are selected by the pilot with the fuel selectors, rather than pumped. The engines are either running from the mains (tips) or the aux (wings), there is no transfer aux to main. Note that if you're flying off the aux tanks, excess fuel from the aux tanks will be returned to the mains, rather than back to the aux tanks from where it originated. If the plane has wing locker tanks, they would be pumped to the mains, then burned from there, though they are comparatively small capacity. There are also pumps which pump the fuel within the tips. So, I suggest confirming exactly the configuration of the fuel system.

Then, more basically, if a fuel pump is not working, or worse "shorting", the plane is unserviceable, it should be repaired before further flight. The fuel pump electrical circuits are simple, so there should be no complex reason they're not working. If the reason is complex, the plane has a bigger problem. For what it's worth, I attended an aging aircraft presentation at Cessna in Wichita many years ago, focusing on the 400 series (but equally applicable to the 300'S). Though the presentation would have been expected to be mostly about structure, the presenter said that major defects were common in electrical systems, and aged electrical systems require extensive inspection and testing for condition, and correct operation.

If you're asking this because your considering operating/owning the plane; the 340 was a wonderful plane in its time. It's now an older plane, and subject to many burdensome inspections. That doesn't make it a bad plane now, but a more burdened, and expensive plane to operate. Thoroughly understand the plane before you accept responsibility for operating costs.
Pilot DAR is offline  
Old 30th Jul 2020, 15:35
  #59 (permalink)  
 
Join Date: Jan 2004
Location: West Coast Canada
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I am current on the Cessna 340

it has up to 9 electric fuel pumps

2 in each tip tank, one to pump fuel from the back to the front of the rank and one to provide fuel pressure to the engine for priming and in the event of an engine driven fuel pump failure

1 in each wing to provide positive pressure and vapour suppression when the engines in feeding from the wing aux tank

1 in each wing to provide pressure to transfer fuel from the nacelle tank (if fitted) to the tip tank

1 in the nose to provide fuel pressure to the cabin heater

The fuel system is very complex and a full understanding of it is essential in order to safely operate this airplane. Depending on what pump has failed it can be a nuisance (eg no heat) to flight safety critical (engine failure)

As a general comment airplanes which have sat for a long time are often maintenance nightmares. I once ferried a Cessna 421 home for its new owner. The aircraft had a similar history to what you described. The snag list when I got it home had 59 items. For the first 6 mon5s it was in the shop after every flight. Eventually after a lot of hassle and expense the aircraft was brought to a reliable state but it was frustrating and painful. Even though the owner got a smoking deal on the airplane he vowed never again....
Big Pistons Forever is offline  
Old 30th Jul 2020, 20:35
  #60 (permalink)  
 
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Many thanks to both of you, much appreciated.
I know next to nothing about the 340 and I don't know the full story of this particular bird so I simply reported what the pilot told me, but it's possible that I misunderstood. I will make sure to study the POH thoroughly next time.

It seems to me that generally speaking it's a very capable airplane, able to operate from grass runways without problems. But obviously bringing it back in line will probably be a financial nightmare. What would the dispatch reliability be, if anything was in order?
DirtyProp is offline  

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