Vmca and flaps
I have about 2000 hrs of instructing with about 50 hrs of that instructing for the ME rating. All the scary moments of my instructing career occurred in that 50 hrs. To assume that the student will always do the rational thing is simply unrealistic. The biggest example was when I was doing a engine failure in the overshoot exercise at altitude. The student established the aircraft (a Pa 34 Seneca 1) in the landing configuration at ref speed and I called "go around" As soon as the student had advanced the throttles to full power I retarded the throttle, the student then panicked as the aircraft started to descend and abruptly applied full back stick
, the airplane instantly and I mean instantly snap rolled and when I took over we were inverted and 40 degrees nose down.
. If I had not also been a aerobatic instructor I would have probably died that day.The student wasn't stupid he was just young and nervous in a much bigger and heavier aircraft than had ever flown......and had a massive cranial rectal inversion, something which he was profoundly remorseful for and cognizant of the implications of his mistake.
That and other examples of student brain fade has made me build in a good margin where appropriate to protect both of us as the student gains initial experience operating ME aircraft.
But back to the question of VMC training. I liken the VMC question to how and why of spin training . Yes I show my students how to recover from a spin but I spend most of my time concentrating on spin recognition. The accident record is pretty clear, almost all spin accidents are close to the ground. Even if the pilot initiated a proper spin recovery the aircraft would still been too low to recover. The failure to recognize that the aircraft was in the slow flight regime and then failure to recognize the aircraft was stalling and then failing to control the yaw at the stall was the cause of most "Spin" accidents. The ability to recover from a fully developed spin. or not is pretty moot, the point of the training should be to not let the aircraft spin in the first place
Similarly when teaching VMC the point is not to determine what the actual VMC for that airplane in those conditions, it is to recognize a loss of directional control during single engine flight and to take immediate and correct action to regain/retain control of the aircraft. This can just as effectively and more safely be done by limiting the rudder to half travel thereby ensuring the loss of directional control occurs at a higher and safer speed.
But I emphasize that for most light piston twins, the aircraft will be over 20 knots below blueline speed if it is near VMC. It will have a considerable negative rate of climb and therefore if the speed is allowed to deteriorate to the VMC value close to the ground the aircraft will likely still crash despite the fact that the VMC situation was recognized and correctly dealt with.
That is why in addition to VMC training I also demonstrate the effect of airspeed below and above blue line on the single engine rate of climb. For most light twins a speed of 10 knots lower or higher than blueline will wipe out the SE rate of climb. This IMO is a more important exercise than the VMC demo.
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Thanks for your informative response Big Pistons (by the way, watch for a PM). I have to concede that aside from the DA-42-L360 (which does not lack for power!) the lightest twin I've flown would be an Aztec, perhaps I have a skewed idea of what a light twin should be expected to do on one engine.
Accepting that my flight test type observations are based on flying Navajos, and bigger - not smaller, I would expect that the aircraft would still climb at speeds slower than blue line, just not at the best rate. Reviewing my Navajo testing, wayyyy slow with one feathered, I was still indicating a 250 FPM climb. I do accept that perhaps the "light twins" won't do this.
Getting back to your aerobatic ME student, I can see the problem - the poor student was just completely out of their element. I remember the first time I flew the turbine DC-3 - completely new thing for me. It honestly took 30 minutes just to get used to it at the basic level, before I started "trying things".
From your perspective (perhaps hind sight being 20/20) is it better for the instructor to demonstrate the whole maneuver first, so the student can warm up to it, and model it?
While taking helicopter instruction, my incredible instructor would often do a maneuver, sometimes even just a quick reposition. I would say to him "I'm going to redo exactly what you just did". The intent being to test if I could make the machine do what I had just seen it done (it must be safe, he just did it!). My instructor soon learned that I would as closely as possible duplicate his maneuver, so if I varied, it was a shortcoming in my skill, and he could spot it - there were no surprises (he also learned to not demonstrate things he did not want me to attempt!). For the few times (one sticks out glaringly) where I imagined what to fly next without a briefing, I was very surely told why I must never do that again, and why! VERY memorable!
I agree very much with the analogy of flying at Vmc and spinning. Somewhere there is a middle ground of EVERY pilot must experience these, then only certain pilots should ever actually attempt them unsupervised. How does a mentor pilot know who's who - every pilot should be able to safely accomplish these maneuvers. After all, the certification requirements generally say "must not require unusual pilot skill or attention".
My mentor (retired Transport Canada test pilot) tells me that I'm to demonstrate stalls in the Navajo, with one engine feathered, and the other at 75%, and I say Huh? He says "you can do it, you'll be fine". That was the first time I'd ever done that, with a total of 5 hrs flying Navajos. The plane was just as he said it would be - fine.
It must be frustrating for instructors to feel to themselves, "I have to give this candidate a ME rating, because they have met the minimum standard", but all the while thinking, "this pilot needs ten more hours training in this plane".
When I test something, and declare that it flies within the standard, can I be sure that a competent pilot can fly it safely?
for most light piston twins, the aircraft will be over 20 knots below blueline speed if it is near VMC. It will have a considerable negative rate of climb
Getting back to your aerobatic ME student, I can see the problem - the poor student was just completely out of their element. I remember the first time I flew the turbine DC-3 - completely new thing for me. It honestly took 30 minutes just to get used to it at the basic level, before I started "trying things".
From your perspective (perhaps hind sight being 20/20) is it better for the instructor to demonstrate the whole maneuver first, so the student can warm up to it, and model it?
While taking helicopter instruction, my incredible instructor would often do a maneuver, sometimes even just a quick reposition. I would say to him "I'm going to redo exactly what you just did". The intent being to test if I could make the machine do what I had just seen it done (it must be safe, he just did it!). My instructor soon learned that I would as closely as possible duplicate his maneuver, so if I varied, it was a shortcoming in my skill, and he could spot it - there were no surprises (he also learned to not demonstrate things he did not want me to attempt!). For the few times (one sticks out glaringly) where I imagined what to fly next without a briefing, I was very surely told why I must never do that again, and why! VERY memorable!
I agree very much with the analogy of flying at Vmc and spinning. Somewhere there is a middle ground of EVERY pilot must experience these, then only certain pilots should ever actually attempt them unsupervised. How does a mentor pilot know who's who - every pilot should be able to safely accomplish these maneuvers. After all, the certification requirements generally say "must not require unusual pilot skill or attention".
My mentor (retired Transport Canada test pilot) tells me that I'm to demonstrate stalls in the Navajo, with one engine feathered, and the other at 75%, and I say Huh? He says "you can do it, you'll be fine". That was the first time I'd ever done that, with a total of 5 hrs flying Navajos. The plane was just as he said it would be - fine.
It must be frustrating for instructors to feel to themselves, "I have to give this candidate a ME rating, because they have met the minimum standard", but all the while thinking, "this pilot needs ten more hours training in this plane".
When I test something, and declare that it flies within the standard, can I be sure that a competent pilot can fly it safely?
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Perhaps I have some learning to do... I am well aware that Vmca maneuvering close to the ground is very high risk, and the cause of too many accidents, particularly when un planned. However, when carefully planned, at altitude, why the worry?
Look at this graph:
You can see that at first you'll hit Vmc. As you climb up you'll reach an altitude that will make the stall the first thing you reach. I'm assuming you know that Vmc will decrease with an increase in altitude because of the lower power created by the operating engine. The stall is the same at any altitude because it's "indicated airspeed".
Hitting Vmc first: this is dangerous! What happens is the aerodynamic requirements of the rudder to counteract yaw will increase exponentially as you approach Vmc. If you reduce speed to 10-15kts above Vmc and then slow at a steady rate (1kt/sec) toward Vmc, you'll notice that you need to add the rudder faster and faster. At Vmc you'll begin rolling towards the inoperative engine and should be at full rudder around that point. You can recover by pitching down, holding full rudder, and rolling the airplane back to level. Nice and simple and the safest! If you approach Vmc at say 5kt/sec, you will be upside down before you know it. As above, the yaw increases exponentially requiring an exponential increase in rudder towards Vmc. As you hit Vmc, you'll be continuing to slow down and hence make the rolling and yawing tendency even greater. This is why the airplane will "snap" towards upside down. If you get a "snap" towards the inop engine, you must reduce the throttle, pitch down, roll out and then apply power as you pitch back up.
Hitting the stall first: a very bad day for you! This is far worse than hitting Vmc first. At a high enough altitude you will hit the stall first as Vmc reduces below the stall speed. When you stall you essentially "fall out of the sky"... work with me for a bit! If you stall both wings at the same time or one slightly more than the other (getting a wing drop) it's a pretty standard recovery and really not much to be worried about especially at high altitude. Now, imaging stalling one and having the other one still flying. You'll roll and yaw towards the stalled wing because of the lack of lift and increased drag. Now strap on a 180hp engine to the flying wing and put that to full power. You will have an extraordinary amount of yaw towards the stalled wing now! It also has the effect of keeping the unstalled wing even further from the stall as it 'blows' the wing creating more lift. What happens is essentially the stalled wing "falls out of the sky" while the flying wing (mostly because of the lift created by the engine) is now accelerating you into a spin. If you think a power on entry to a spin in a single engine airplane makes the spin more "fun" or "intense", what do you think would happen to that spin when you have the engine running at full power, and located a few feet from the longitudinal axis?!
Regardless of where Vmc is, hitting the stall first is incredibly dangerous. At the point on the graph where Vmc intersects the stall speed would be the worst case scenario. As you increase in altitude from that point the effects aren't as bad, even though the graph might make you think it is so. The "recovery might be difficult" pointing towards the area where the stall is encountered first is true when compared to the area where Vmc is hit first, however, make sure you know that the worst case is at the intersection point!
I want to mention some things with regard to published stall speeds. The BE95 has a published Vmc of 80mph and a stall speed of 85mph. Those numbers are at sea level. To begin with, Vmc is lower than the stalling speed, and even though hitting a stall while single engine can be catastrophic, I prefer to have the stall first. A basic reason why is because I know what the stall speed is regardless of altitude, Vmc is always changing. With regard to the twin engine stall with power on, you'll get a stall speed lower than published, quite significantly lower actually. The POH states that with 25" and 2700rpm, the twin engine stall speed is 61mph with gear and flaps up. A difference of 24mph! It's more like a fun fact though, as being that slow will be below Vmc significantly at the majority of the airports around North America. But what about the single engine stall speed?! The answer is, it doesn't really change. It will be essentially the same as the published stall speed for the configuration. When an engine fails, it's similar to pulling the throttle to idle. If you were to be very technical, in some cases, the single engine stall speed could be slightly higher than the published stall speed because it is producing zero lift and more drag than at idle (assuming a non feathered position). So, like we discussed above, the inop engine's wing will stall at the published stall speed while the operating engine's wing will still be flying.
Pilot DAR, you asked if this training could be carried out safely and I think it can and I think it should be carried out. However, due to all the complexities and dangers of Vmc training, it's imperative that the candidate and especially the instructor, have a thorough understanding of what can happen and why.
Last edited by italia458; 26th Oct 2011 at 18:29.
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Accepting that my flight test type observations are based on flying Navajos, and bigger - not smaller, I would expect that the aircraft would still climb at speeds slower than blue line, just not at the best rate. Reviewing my Navajo testing, wayyyy slow with one feathered, I was still indicating a 250 FPM climb. I do accept that perhaps the "light twins" won't do this.
It must be frustrating for instructors to feel to themselves, "I have to give this candidate a ME rating, because they have met the minimum standard", but all the while thinking, "this pilot needs ten more hours training in this plane".
Regarding your stall in the Navajo: What made it manageable is that you were probably at an altitude that is significantly higher than the Vmc/stall intersection and were at reduced thrust, further reducing Vmc. Even at the point where Vmc/stall intersect, I think a skilled pilot could easily recover assuming that he is ready and waiting for the first indication of a stall/roll/yaw to retard the throttles and complete the rest of the recovery. The reason I don't openly recommend it and why most documents these days do not recommend it is because of how quickly it can get out of hand if you're not ready at the instant it requires input to recover.
I have no idea what your skill level is so I would just recommend that you be careful. Being able to recover easily from the single engine stall at 75% power in a Navajo might have been a bad experience for you if you now think that it's a non-event. The difference between an easy recovery and being out of control is a somewhat thin line.
Accepting that my flight test type observations are based on flying Navajos, and bigger - not smaller, I would expect that the aircraft would still climb at speeds slower than blue line, just not at the best rate. Reviewing my Navajo testing, wayyyy slow with one feathered, I was still indicating a 250 FPM climb. I do accept that perhaps the "light twins" won't do this.
From your perspective (perhaps hind sight being 20/20) is it better for the instructor to demonstrate the whole maneuver first, so the student can warm up to it, and model it?
I can also assure you when introducing a new training manoever, I don't just tell the student "hey why don't you give the engine failure in the go around a try and see how you do"
The manoeuver was properly and fully briefed on the ground and demo'd by me in the air. In fact it was the students second try at the manoever. I want to be clear I was PIC so this upset was ultimately my fault and not the students. Obviously there was a big post incident debrief with the CFI and the other instructors. The take aways were as follows.
1) On the first try I had briefed that the "airfield elevation" was 3500 feet AGL and that the go around would be called at 3700 feet. He was too tentative rotating the aircraft to a nose high attitude and we lost 300 feet. I pointed out he went 100 feet below ground which doesn't work very well. This I think lead to the over aggressive pull back on the wheel as he was responding to my feed back, also
2) The Seneca has a very heavy elevator and I had not noticed that the student had applied a lot of up elevator trim with the electric trim button. He was expecting to need the usual big heave on the wheel and so the much lighter control pressure contributed to the over control and excessive up elevator. and
3) Instead of immediately taking control I tried to talk the student into correcting the problem. This was a bad mistake as he was task saturated and what I was saying was not penetrating, and finally,
4) I was unprepared for how fast the aircraft lost airspeed with a strong pitch up at Vref (90 mph) gear down and flaps full on one engine. The stall horn/actual stall/and violent roll/yaw into the dead engine all happened in maybe 2 seconds.
The only good news is I can personally attest that, on the Seneca at least, the gear retraction time is cut in half if you select gear up when the aircraft is inverted
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The only good news is I can personally attest that, on the Seneca at least, the gear retraction time is cut in half if you select gear up when the aircraft is inverted
...maybe with a little negative G to help!
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Yes, Big Pistons, I was pleasingly surprised by the performance apparently available to me in the Navajo I was testing. Having read the flight manual for it, I completly agree that there are other conditions of flight, where it would be nowhere near as good! Fortunately, I'm ususally able to control many variables to make them favourable for my testing - I'm not relaly very bold in planes anymore!
I can imagine the startled student syndrom. Though I am not an instructor, I do find myself doing checkouts is certain waterborne aircraft from time to time. As you well know, the water is unforgiving! I dread the day when I have to check out a new owner with limited taildragger experience on the amphibian I'm selling for it's owner. You gotta let the pilot get a little out of control, so they see it happening, but this plane does not have far to go, before it's gone too far! It's a greater challenge to instruct than most pilots realise. While taking confined area trianing north of Pitt Lake, my mentor pilot apologized in advance for guarding the controls so closely, say that there just would not be time to reach if he had to - fine with me! But I drift....
It is always of interest to me, as to what I should be writing in flight manual supplements, so as not to be too wordy (I save that for PPRuNe), yet convey the intent that a pilot approach certain maneuvers with caution, or preparedness for an unexpected outcome. No one expects a Seneca to be on it's back. Getting there is probably not difficult, getting back would be much more exciting
I can imagine the startled student syndrom. Though I am not an instructor, I do find myself doing checkouts is certain waterborne aircraft from time to time. As you well know, the water is unforgiving! I dread the day when I have to check out a new owner with limited taildragger experience on the amphibian I'm selling for it's owner. You gotta let the pilot get a little out of control, so they see it happening, but this plane does not have far to go, before it's gone too far! It's a greater challenge to instruct than most pilots realise. While taking confined area trianing north of Pitt Lake, my mentor pilot apologized in advance for guarding the controls so closely, say that there just would not be time to reach if he had to - fine with me! But I drift....
It is always of interest to me, as to what I should be writing in flight manual supplements, so as not to be too wordy (I save that for PPRuNe), yet convey the intent that a pilot approach certain maneuvers with caution, or preparedness for an unexpected outcome. No one expects a Seneca to be on it's back. Getting there is probably not difficult, getting back would be much more exciting
