Why does Vmc decrease with altitude?
Thread Starter
Join Date: Jan 2010
Location: India
Posts: 139
Likes: 0
Received 0 Likes
on
0 Posts
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?
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?
Last edited by DJ Flyboy; 14th Feb 2011 at 00:36.
Join Date: Oct 2007
Location: fort sheridan, il
Posts: 1,656
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Thread Starter
Join Date: Jan 2010
Location: India
Posts: 139
Likes: 0
Received 0 Likes
on
0 Posts
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
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
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
Moderator
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.
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!
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!
Join Date: Apr 2010
Location: Home soon
Posts: 0
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Sep 2004
Location: UK
Posts: 12
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Nov 2004
Location: Here, there, and everywhere
Posts: 1,124
Likes: 0
Received 12 Likes
on
7 Posts
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?
If so, does the propeller push back as much air at FL180 for the same horsepower?
Moderator
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.
.. 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.
Join Date: Oct 2010
Location: New Zealand
Age: 34
Posts: 88
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Oct 2010
Location: 5° above the Equator, 75° left of Greenwich
Posts: 411
Likes: 0
Received 0 Likes
on
0 Posts
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?
But what about jets and turboprop engines?
Join Date: Nov 2007
Location: United Kingdom
Age: 71
Posts: 713
Likes: 0
Received 0 Likes
on
0 Posts
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!
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!
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.
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.
Join Date: Nov 2007
Location: United Kingdom
Age: 71
Posts: 713
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jul 2009
Location: France - mostly
Age: 84
Posts: 1,682
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by TheChitterneFlyer
there comes a time when you must reduce your speed below Vmca(2) towards Vref.
regards,
HN39