Why does Vmc decrease with altitude?
 

Im currently doing my multi rating and am completely confused when it comes to Vmc
Why does Vmc (indicated) decrese with an increase in density altitude?

As per my knowledege Vmc is the minimum speed at which directional control of the A/C can be maintained with one engine failed, max power on the operating engine and 3-5* bank away from the failed engine. Vmc has nothing to do with climbing or decending..only directional control.

My explaination to the above question is that with the thrust reducing with altitude, the moment decreases and therefore less rudder input is required to maintain directional control, but it was shot down by one of the instructors, he said that the rudder effeciency too will decrease with altitude.

Another question that I have is if i am producing 'X' thrust at 5000 feet and 10000 feet, will my IAS be same? (The TAS will increse (right?)) or in other words..is less thrust required at 10000 feet as compared to 5000 feet to maintain the same IAS?

I don't think the rudder decreases in efficency with altitude PROVIDED YOU ARE AT THE SAME INDICATED AIRSPEED.

IF I WERE YOU, I would find a flight manual for a turbocharged plane and see the graphs for Vmc(a) and see for yourself what it does, as a turbocharged engine shouldn't lose power with altitude.

For example if you are learning to fly in a normally aspirated Piper Seminole, compare the graphs for a Turbocharged Seminole.

Ihope you will post what you learn.

I have to admit, I was a CFIIMEI back in pre!!!!oric times and I fly jets now, (737), and haven't really thought about Vmca for awhile. I stay away from it!!!!

Also, one other thing...no matter what the book says the speed is, when you start to lose control, you are at Vmca, so get the nose down and reduce power on the operating engine...don't even get close to VMCA at low altitude as this part of multi engine training is one of the most dangerous . One engine go arounds in light twins are very dangerous too...so BE CAREFUL

Will do thanks :ok:

Think your instructor is wrong - I have had many "expert" opinions which have been utter rubbish.
Vmca demonstrations are extremely dangerous.
I have only done one - started at 300ft over the sea (as pointed out on a naturally aspirated engine density altitude effects it) - we selected zero thrust on the critical engine and I reduced speed - I had on full rudder , opposite aileron and the stick on the back stops with the stall warning going before I lost directional control.
On the ground I argued with my instructor that it was a dangerous exercise - I had been educated far more extensively - he had been a hurri bomber pilot and had left a wing tip on a tree stump in the far east in 1944.
The college stopped the exercise a few years later - there had been several fatalities in the states.
The RAF lost more meteors flying SE exercises than due to real engine failures.
And BEA airtours lost a 707 doing another stupid engine failure exercise.

There are a lot of ill informed people in aviation - read as much as you can including accident reports - and hopefully you will avoid repeating other peoples mistakes.
good luck

Why does Vmc (indicated) decrese with an increase in density altitude?

main driver is thrust. However, be aware of the differences between a normally aspirated and turbo/super-charged installation

max power on the operating engine and 3-5* bank away from the failed engine

You might be confusing things a little. Vmca usually involves a restriction on the OEM to limit bank to 5° so, unless you have specfic Type information that a lesser bank angle was used, presume 5°.

For OEI climb, the best performance generally is somewhere around 3°. This will have little relevance to Vmca.


full rudder , opposite aileron and the stick on the back stops with the stall warning going before I lost directional control

obviously this happened a long time ago .. but how does one spell "spin" ? Unfortunately, the military lost a lot of folk during training mishaps rather than due to combat.

Sounds like you are fortunate to be with us still and be able to tell the tale.

yup but lost a few friends on the way.

Worked for an airline who destroyed 8 aircraft in the six years I flew with them.

Bar one all were our fault.

Lost my best friend in the second one - we operated the aircraft contrary to it's design philosophy and to safeguard the idiots they back dated a previous non existing stall recovery procedure for the inquiry.

Got out as soon as I could and have never regretted it, and have spent fifteen years of my spare time teaching amateur pilots - not accepting renumeration allows one to tilt the windmills and hopefully make aviation a little bit safer!

One thing I did learn is that the more bloggs knows the more devious ways bloggs can invent to try and kill you!

Less thrust,therefore less assymetry,less yaw ,lower vmc.
Assume temp higher,less thrust,less yaw,vmca in air lower,v2 minimum is less therefore your v2 speed will be less.v2 is directly linked to vmca which is linked to thrust.

I don't know the answer but I question the ralationship to thrust.

My aircraft has a declared sea level Vmca which reduces by some 1kt for every 1,000ft above sea level. The engines are flat rated to around 18,000ft.

BIG

Flat rated meaning that you are putting out the same horsepower to FL180?

If so, does the propeller push back as much air at FL180 for the same horsepower?

Last time I looked it didn't have a propeller but, yes, it is producing the same amount of thrust to a relatively high altitude...

BIG etc.

v2 is directly linked to vmca which is linked to thrust.

.. and stall speed. Characteristic result is a notionally constant V2 at very low weight and then an increasing V2 when stall becomes the controlling consideration.

According to the very cheesy Jeppesen Multi engine video Vmca decreases with altitude because the operating engine produces less power at higher altitudes. This is likely assuming a non turbo engine because some turbo engines are rated higher at altitude because the air charge is cooler but they can maintain the same manifold pressure (thanks turbo!!)

My logic is telling me that Vmca could increase with altitude on a turbo charged aircraft for this reason but please correct me if I am wrong

You're missing something here. I've never heard of a normally-aspirated or turbo-charged jet engine. Funnily enough, they also have Vmca limitations.

G'day ;)

Yes john,

1.13 vs or 1.1 vmca is minim v2.

Well, we would have to take into consideration the propeller (for the engines equipped with one at least). The engine may give the same power output, but the propeller is still affected by the conditions of a less dense atmosphere at higher altitude, so it won't move the same amount of air it would at sea level thus producing less yaw anyway.

But what about jets and turboprop engines? :confused:

As John Tullamarine has alluded to Vmca is directly proportional to engine thrust.

Think of it in this way... in very cold conditions an engine (jet or piston engine) will produce its maximum (red-line) power. In very hot conditions it will produce considerably less power. Therefore, the power output of the "live" engine is dependant upon the ambient temperature.

Considder the following examples for both hot and cold scenarios:

Cold ambient.
With, for example, your left engine failed, the remaining "live" engine will still produce its red-line maximum thrust. This will produce a yaw towards the "dead" engine. Associated with that yaw there will be a minimum speed for maintaining directional control of the aeroplane by the use of rudder. This is termed Vmca; minimum control speed in the air.

Hot ambient.
With the left engine failed, the remaining "live" engine will produce less thrust than in cold ambient conditions. This will produce a yaw towards the "dead" engine; however, in hot ambient conditions, the ammount of yaw (turning moment) will be reduced and that less rudder will be required to control the aeroplane. In this example, the lowest speed at which you can still control the aeroplane is now reduced, because you will have more rudder available to control the yaw (less thrust from the "live" engine = a reduced turning-moment).

So, Vmca will reduce as ambient temperature increases. Or, if you prefer, Vmca will increase as the ambient temperature decreases... it's the same statement.

Demonstrating Vmca is NOT for the feint-hearted; nor should it be demonstrated by anyone other than a very experienced instructor; also, that he is VERY familiar with the particular aircraft type.

Generally speaking, Vmca will decrease by 1-2 kts per 1000 feet. Also, Vmca will decrease by a similar amount for each 10 degree increase in ambient temperature. Do check your own Pilots Operating Handbook for exact figures.

All of the above is a simplistic method of understanding Vmca. The complexities about whether it's a turbo-charged piston, a normally aspirated engine or a jet engine (or turbo-prop) will only add to your confusion. Keep it simple.

I hope this helps

TCF

Carrying out a Vmca demonstration at 300 feet over the sea is, in my humble oppinion... seeking a death wish!

@ Morrisman 1

According to the flight manual, yes Vmca does increase slightly with altitude on a turbo charged aircraft!

(Seneca 2)

Your instructor was very dumb or very brave!!!!!!!!

Back to the original question

Vmc is dictated by rudder authority and the amount of asymmetric thrust. For any given condition e.g. weight, C of G etc, rudder authority is determined by IAS. For a normally aspirated aircraft max thrust is determined by air density. Therefore on a normally aspirated aircraft as density decreases with altitude so will max thrust therefore Vmc will also decrease as less rudder auhority is needed to counter the asymmetric thrust.

I agree that Vmc needs to be done with caution by those that know what they are doing. It's not the Vnc demo that counts, it's knowing and carrying out the correct recovery technique that counts. A bit like stalling, are we teaching stalling or stall recovery technique?

When ever I teach Vmc recovery I block the rudder travel with my foot on the lazy (dead) engine side. This artificially raises Vmc abut also allows for further rudder travel if needed should the recovery be poorly executed.

From a training point of view it is irrelevant what IAS Vmc occurs at, it is the application of the correct recovery technique that counts.

One thing to remember is Vmc is calculated at the rear most C of G and at sea level. Most training aircraft at not being operated at anything like the rear most C of G or at sea level therefore on some aircraft the actual Vmc on the day may be quite close to the stall speed.

Indeed, the correct recovery technique is all important; equally important is an understanding of what's going-on during the final approach phase.

In large multi-engined aircraft i.e. B747 or similar (with 2 engines out), there comes a time when you must reduce your speed below Vmca(2) towards Vref. Should it then (subsequently) become neccessary to carry out a Go-Around you cannot simply apply max thrust and point the aeroplane away from the ground. Your speed will be below Vmca(2) and you will lose control of the aeroplane. Therefore, you must continue descending whilst "cleaning-up" and accellerating (increasing thrust commensurate with maintaining control of the aeroplane). When Vmaca(2) is achieved it's only then that you can apply max thrust and start to climb away from the ground with a slight bank towards the live engines (the exact bank angle is published within the Flight Manual and it will be of the order of 3 to 5 degrees).

It's therefore vitally important that you're "aware" of what that particular speed actually is and that you keep it somewhere in the back of your mind for the Go-Around case. Carrying out a double assymetric Go-Around certainly concentrates the mind and that it will demand "precise" handling of the aeroplane. To alleviate the Go-Around case you should considder (in advance) asking ATC for a sterile runway; particularly when you're committed to land i.e. when you've reduced your IAS below Vmca at, say, 1500 feet.

A Go-Around is usually neccessary due to another aircraft being on the runway; thus preventing you from landing. Many times that I've been in the simulator (and carrying out such procedures) it's usually the case that the instructor will induce the double assymmetric Go-Around... it's part of your licence requirement. However, in the real world, if you're making your final approach (committed to land) and the controller says "Expect late landing clearance due departing traffic"... considder this... if you can see that the depature traffic is well on the way to getting airborne; go ahead and land! Why? A Go-Around from three or four hundred feet will result in disaster. It's better that you're on the ground and taking to the grass than trying to fly an impossible procedure that will only result in a pile of mangled aluminium somewhere outside of the airfield perimeter.

Take care y'all

TCF

Be aware that on a Turbo-Charged engine, Vmca will slightly increase as you climb. This is due to the fact that the turbo controller is sensing temperature and pressure (density) and will apply "boost" in order to maintain power with altitude gain.

At what weight does VREF become less than VMCL-2? When conducting an approach with two engines out , would it perhaps be better to select the longest available runway and not reduce speed below VMCL-2 ? Would you accept "Expect late landing clearance due departing traffic" in that situation?

regards,
HN39

John T is correct...
 

D J Flyboy,

There are fundamentally two considerations with regard to this issue. First is CONTROL. The second is PERFORMANCE.

Vmc will decrease with altitude in a normally aspirated engine due to the fact that the engine is producing less thrust. Therefore, your control surfaces, at a given speed, will be better able to counteract the yaw tendancy due to a failed engine....when the remaining engine is producing less power. There will be less yaw into the failed engine, because the operating engine is producing less thrust.

Performance is related to the 5 degrees of bank issue. This has to do with sideslip, drag (DRAG is the key word....as, with an engine out, there is less thrust to overcome the drag. This is a performance issue, not a control issue.), and slipstream, fuselage drag, etc.

To address the comment about 300 feet AGL, stalling, etc., John T is correct: "Can you say 'SPIN'?

To explain: Usually, at lower altitudes, where the engines produce their best thrust (denser air, more air), the aircraft control is Vmc limited. That is to say, the aircraft will reach its Vmc and roll over on its back before the wings stall.

At higher altitudes, the Vmc is lower and will approach the stalling speed of the wings. At some point, with low power on the operating engine(s), the power output is little, and the wings will stall (You will reach the stall speed, as the plane decelerates, before you run out of aileron and rudder.)

So, when you do this training stuff, you either stall first, or roll first. All is well.

But, wait, there's more. What happens when the roll speed and the stall speed are the same? Can you say 'SPIN'?

And, this is what gets people in trouble. In any aircraft, stalling in uncoordinated flight will induce a spin.

So, you've lost weight; you started exercising, quit smoking, etc., and your life longevity is increases....maybe by many years.

But, when messing around with a multi-engine plane, hitting the Vmc and stall speed at the same time, your life longevity is now reduced to seconds. It's unlikely you'll recover. Especially with a C.G. that is aft of very forward.....

There are many sources of information that will explain this to the new, multi-engine student. Research, have a read, then you'll understand.

Fly safe,

PantLoad

P.S. Please don't do stalls or Vmc demonstartions at 300 feet. This is downright stupid.


(Edited to add the post script....)

I remember when the Piper Seminole came out and it was a big deal that stall was pretty far away from Vmca

HazelNut39, I no longer have my Perf Manual to give you that information; however, the Boeing Flight Training Manual will provide a similar quote...

[FONT='Arial','sans-serif']

Quote:

---

[FONT='Arial','sans-serif']During an approach with two engines inoperative on one side it may be possible to be at an airspeed below VMCA2 when a go-around is initiated. In this event, thrust should be applied with rudder application and a slight bank into the operative engines while establishing a descent for faster acceleration. Stop the thrust lever advance just prior to full rudder travel and then set the inboard engine to go-around thrust. As the airspeed increases and excess rudder becomes available, advance the thrust lever for the operable outboard engine to the go-around setting. At bug + 60 select flaps 1 and slowly increase the pitch attitude to maintain bug + 60 and transition from a descent to a climb.[/FONT]

---

Let's not get wrapped-up in semantics... the point being is that you should be aware of what the actual Vmca(2), or, Vmcl(2) should actually be. It's one thing to sit comfortably in an armchair at home and disect what someone should have done when in fact you only had a few seconds to make a sensible judgement at that moment in time.

If you're not able to put the aeroplane into a safe Go-Around configuration and that the current situation allows you to land... land the sucker! I'd much prefer to be climbing out of an intact aeroplane than a pile of wreckage somewhere outside of the airfield perimeter.

The chap who was practicing recovery from single engine work (at 300 feet) was obviously well below Vmca; which is why he had difficulty in making a recovery from what should have been a total disaster!

TCF[/FONT]

It is sobering to review the circumstances of the RAAF mishap with B707 A20-103 at East Sale in Victoria some years ago. An extreme set of circumstances involving aspects of inappropriate operation .. with essentially predictable results. Well worth a read for multiengine instructor pilots who fancy that their flying skills are up there with the aces.

One needs to keep in mind that book Vmca is quite specific, the real world figure rather variable according to configuration and ambients. While there may be a case for exposing a pilot in training to general aspects of Vmca, I suggest a better option is to stay away from it and let the TPs come up with the numbers for the AFM.

If you MUST play with Vmca, do it with consideration, sufficient height to provide for recovery if it gets away from you .. and be prepared to close the other throttle to get rid of the yawing moment.

I don't agree entirely

As I understand things it is a control issue as well. The up to 5 degrees of bank requires less rudder input, therefore assists in controlability.

However we demonstrate the effect of the 5 degrees bank angle by showing the difference in RoC, with and without the bank angle applied.

27/09
 

27/09:

You are correct. I was wrong. Banking into the live engine does, in fact, both improve climb performance, but also lowers Vmc. You are correct.

I had to dig through my references...dust off the books....

The 'ball' is valid on a multi-engine engine only where there is symetrical thrust. With one engine inoperative, centering the ball does NOT provide zero sideslip. Banking the airplane a bit (up to 5 degrees max as per authority limits) with the ball slightly outside of center will not only provide better control, but better climb performance, as well.
'Estimating' zero sideslip will improve climb performance. Again, control is improved, as well.

Thank you for the correction....


Fly safe,


PantLoad

Pantload

The whole asymmetric flight principles of flight subject is a bit confusing at times, especially if you don't teach it regularly. At least it is to me and I consider myself to have a better understanding than most when it comes to principles of flight. I think some of the posts on here prove how many fish hooks there are in this subject.

It always seems to provoke interesting debate when ever it gets a run on a forum like this.

Important to put handling and climb performance in two adjacent paddocks.

(a) Vmc - VERY bank dependent. Not hard to find aircraft with a 30-40kts increase in Vmca if you bank the "wrong" way. The certification limit of 5° puts a constraint on the innovative OEM. I suggest that, if the book doesn't say different, presume that the AFM Vmca figure likely is based on 5°. Back near Vmca one is interested in not dying immediately so handling is the name of the game - performance sits in the wings at this time.

(b) climb performance OEI will be maximised somewhere near the condition for zero side slip (think about it - what's the easiest way to generate a bunch of drag and an increase in descent gradient in a light single overshooting the runway on approach ?)

It turns out that zero side slip occurs around 2-3° bank into the operating engine(s) for pretty well all multis (337 etc notwithstanding). Given that flying this sort of bank angle is demanding on the pilot (and, for some aircraft, can't be done due to AH limitations) the oft-chosen compromise is to fly wings level and accept the small climb penalty. It follows that 5° bank is going to be somewhere near the climb capability found for wings level so there is no point in trying to climb at 5° once you have the aircraft established in the climb at an appropriate target speed in excess of Vmca. If the aircraft is light enough to achieve an impressive OEI climb, go for wings level anyway.

(c) Vmca region - go for 5°. Established in the OEI climb wings level makes good sense and, if you REALLY want the last bit of climb (and you are smooth enough to do it well), go for 2-3°