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For Jet Or Others Re Vmc  
User currently offlineZionstrat From United States of America, joined Apr 2001, 226 posts, RR: 0
Posted (11 years 9 months 3 weeks 4 days 15 hours ago) and read 3908 times:

In the midst of the Charlotte tread,

http://www.airliners.net/discussions/general_aviation/read.main/1000464/

I had the following discussion with Jetpilot, but considering I was off topic, I’m not sure we ever got to the root of the question- I’m trying to understand the differences between VMC on light ac and commercial and how VMC is affected per my final note.

Figured I’d move this over here to see if I can get a better understanding-
Thanks in advance-


Topic: RE: Charlotte Crash
Reply: 49
Username: Zionstrat
Respect Rating: 6
Posted 2003-01-08 17:06:43 and read 2939 times.

Dragogoalie got it right- Below VMC you have 3 choices:

Trade height for airspeed (not an option on take off)
Reduce power on the good engine and glide in
Roll out of control

Again, it's way too early to be certain, but it sounds like classic twin failure on take off-

Those of you who pray, don't forget the families.

Username: JETPILOT
Respect Rating: 71

Posted 2003-01-08 17:18:11 and read 2862 times.
Zionstrat..... There are no options below VMC... Below VMC your dead. This plane was designed to be able to perform in the worst case senario by relation to VMC, and still be able to climb. Overwhelmingly the chances were they were not in the worst case scenario, and if they were the plane has demonstrated it is still poosible to climb sefely.

JET

Username: Zionstrat
Respect Rating: 6
Posted 2003-01-08 17:38:58 and read 2691 times.
Jetpilot-

"Zionstrat..... There are no options below VMC... Below VMC your dead.."

Could you expand on this theme as you seem to have made my point? If they lost an engine and for whatever reason fell below VMC, there aren't any good options on takeoff unless there happens to be lots of runway in front and you cut power immediately.

You point out that "This plane was designed to be able to perform in the worst case scenario by relation to VMC" and I agree if you mean is that it's pretty difficult to get below VMC in this ac with proper training. But again, if they were below VMC the result would be very similar to what happened. Am I missing something?

Username: JETPILOT
Respect Rating: 71
Posted 2003-01-08 17:52:27 and read 2600 times.
Zionstrat...

Your application of VMC applies to light aircraft that do not have to demonstrate a rate of climb on one engine.

During certification the 1900 and all planes in use for carrying pax must show a rate of climb in the worst circumstances.

Ex... worst case.... Aft CG, @MGTOW, high altitude, high temps, flpas down, gear down.

So going below blue line which is the mimimum safe operating speed with one engine innoperative doesnt apply because the plane already demonstrated it can climb under available power.

JET

Username: Zionstrat
Respect Rating: 6
Posted 2003-01-08 18:19:51 and read 2425 times.
Jetpilot-

You nailed my description since I fly nothing but light ac, but I'm still not sure I follow- For the sake of learning, let's say you bust Vmcg-If directional control can't be assured on the ground, how could it possibly exist if you manage to get airborne? Or are you saying that Vmcg doesn't apply to commercial ac?

If that is the case, commercial ac rudders must be able to compensate for asymmetric thrust in any configuration or speed down to Vso? I'd appreciate any sources that might help me understand this better.

Thx
z


14 replies: All unread, jump to last
 
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 1, posted (11 years 9 months 3 weeks 4 days 9 hours ago) and read 3878 times:

It's a little complicated to follow but here's what I think your asking...

An engine failure below VMCG is an abort. VMCG must by certification be a certain precentage below VR.

If the AC falls bellow VMC you are now fighting for survival. Depending on conditions the indicators of VMC, are loss of directional control, or the first indication of a stall. At that point your below VMC, and you will be trying to save the aircraft by lowering the nose and simultaneously reducing power enough to get back to blue line which is MSSE Minimum safe single engine speed which has a built in fudge factor from VMC.

If your in IMC, and lose an engine oyur visual clues to the yawing arent there. So you have to look at your ball... engine instruments might not give an immediate indication of which engine you lost.

If your at a high angle of attack as in a departure you have to lower the nose and maintain airspeed. In light twins it might not be able to maintain a rate of climb, and be at a safe airspeed. So a descent might be the only way to hold airspeed above VMC. Light twins do not have to show a rate of climb on a single engine.

Below VMC the plane is in the process of inverting and entering a flat spin. Immediate action is required. If you let things deteriorate to the point where your entering VMC then your situational awareness is essentially zero, and your going to die.

Was this info on the right track?

JET



User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 2, posted (11 years 9 months 3 weeks 4 days 9 hours ago) and read 3867 times:

In addition ... in large aircraft there is no MSSE... after departure its V2. Takeoff safety speed your shooting for in the event of an engine failure.

JET


User currently offlineAAR90 From United States of America, joined Jan 2000, 3477 posts, RR: 46
Reply 3, posted (11 years 9 months 3 weeks 4 days 6 hours ago) and read 3864 times:

Zionstrat,
I think I'd start by simplifying your definition of VMC. Eliminate all the regulatory definitions (too complex for a simple minded pilot anyway) and just understand that VMC is nothing more than the speed at which an airplane may still be flying, but the pilot no longer has directional control over where it is going. Slower than VMC and the pilot can not control where the plane is pointed. Faster than VMC and the pilot should have directional control. Actual VMC speed varies with the situation (i.e. the single published number is only good for the pre-defined circumstances). Anything else adds some other flight characteristic that complicates the situation, but does nothing to change VMC.

I used to demonstrate VMC flight in the E-2C Hawkeye simulator at Miramar in my instructor days. When all was done, and if I had a good student pilot, that student would find himself at maximum power (5600 HP) on one engine, drag on the other (-1500 HP), maintaining level flight (600' above water/ground), and in a slow (usually right) turn with full aileron and rudder deflection. That is VMC flight. Speed would be anything up to 25 knots below VMC --at which point the plane will stall. The observed turn rate would vary as the aircraft's speed varied.

Imagine trying to fly a plane (any plane) in that situation and you'll understand VMC. Vary anything and you'll be introducing a different flight characteristic (not VMC) to the situation. How do you escape from a VMC situation? You must increase airspeed to increase the effectiveness of your flight control surfaces.

In the above situation I created there was not enough altitude to trade for enough airspeed increase to regain directional control. The only option for the student pilot was to pick the best looking site for a controlled(sic) landing/ditch. Had to be underneath the circle in the sky he was drilling though. Oh how I miss those instructor days.  Smokin cool



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 4, posted (11 years 9 months 3 weeks 3 days 22 hours ago) and read 3831 times:

AAR90.. "Anything else adds some other flight characteristic that complicates the situation, but does nothing to change VMC."

Can you elaborate on that statement?

JET


User currently offlineAAR90 From United States of America, joined Jan 2000, 3477 posts, RR: 46
Reply 5, posted (11 years 9 months 3 weeks 3 days 7 hours ago) and read 3816 times:

Can you elaborate on that statement?

Sure, by way of example:

Depending on conditions the indicators of VMC, are loss of directional control, or the first indication of a stall.

Indication of an actual VMC condition is the loss of directional control ONLY. "First indication of a stall" is an indication of a stall, not a VMC condition. The two are not directly related.

Sec. 23.149 Minimum control speed.
[(a) VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.


There is no other flight condition (stall, spin, departure, etc.) related to an aircraft's VMC. It is what it is. Whenever encountering a VMC condition one would expect to encounter (pretty soon) other flight conditions (departure, spin, etc.) since they are normally found relatively close together in speed, but that means you'll have a compound problem, not just a VMC problem.




*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 6, posted (11 years 9 months 3 weeks 3 days 6 hours ago) and read 3814 times:

Thanks,,,,

JET


User currently offlineFlightSimFreak From United States of America, joined Oct 2000, 720 posts, RR: 0
Reply 7, posted (11 years 9 months 3 weeks 3 days 1 hour ago) and read 3802 times:

I'de like to throw something in here and say that the B1900 has an autofeather system. I talk with a 1900 pilot every monday, and we go over systems in the plane (I work with him... I am a line person and one of my duties is unloading the UPS Ameriflight flight... I talk with the pilot quite often)

Anyways, if you are saying that the plane had an engine failure, I say no. The pilot told me that if an engine fails, the plane basically flies as if nothing happened. The other engine is more than powerful to fly the plane by itself (This is on the C model, so it might not be correct for the D model, but I think that it would not be that different)


User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1655 posts, RR: 1
Reply 8, posted (11 years 9 months 3 weeks 2 days 8 hours ago) and read 3787 times:

Bear in mind that some light twin piston A/C will automatically condemn you to a liftoff substantially below VMC. That means that the airplane will start flying before you hit VMC. The ONLY choice that you have when flying in that speed range is to cut the good engine and land straight ahead. You will notice that, on takeoff, many light twin pilots flatten out above the runway and yank the gear up immediately in order to accelerate to above-VMC.

The only place where this might not be the case is the airport on Catalina Island; a cliff at the end of the runway lets you dive to get to VMC.


User currently offlineJETPILOT From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 9, posted (11 years 9 months 3 weeks 2 days 6 hours ago) and read 3790 times:

Thirtyecho...

I disagree. You only reduce the power and lower the nose enough to maintain directional control.

Only sometimes does that mean a descent.

And the theory of accelerating in ground effect is not a good idea. For one it's not a procedure in the POH. Second I'd rather have the altitude any day, as this can be traded for airspeed.

I saw a guy in an Twin Commander dig himself a hole in Lakeland after losing an engine about 500 feet off the ground, and trying to circle back top make the field. He reached VMC in the turn, and he became a spinning lawn dart.

JET


User currently offlineZionstrat From United States of America, joined Apr 2001, 226 posts, RR: 0
Reply 10, posted (11 years 9 months 3 weeks 1 day 16 hours ago) and read 3790 times:

Good info- Thanks for the input-
z


User currently offlineB747Skipper From , joined Dec 1969, posts, RR:
Reply 11, posted (11 years 9 months 2 weeks 9 hours ago) and read 3805 times:

A few facts about VMC for transport (jet) aircraft - certified FAR/JAR 25...
xxx
In jet aircraft, the L or R outboard engines failure are the base for VMC speed certifications... there is a difference between VmcG (ground) and VmcA (air) - merely because, on the runway (ground), airplane cannot be banked to assist in directional control... Therefore, VmcG is ALWAYS higher KIAS speed than VmcA is... - VmcG is always the LOWEST possible V1 speed...
xxx
On the 747 as an example - the engine power has also tremendous effect on VMC speeds - for the same aircraft, VmcG for low powered JT9D-7A aircraft is some 118/119 KIAS, whereas VmcG for higher power JT9D-7Q would be 132/133 under same conditions... thereby their different minimum V1 speeds...
xxx
The length of fuselage also affects VMC - as an example, compare, with same engines, the DC8-62 and DC8-63, the longer fuselage -63 has a lower VMC...
xxx
With jet aircraft - there is no L or R "critical engine" - since there is no "P factor" with jet engines... on propeller airplanes, L/R critical engine depends on the rotation of propellers - clockwize or counterclockwize...
xxx
If an aircraft has a stall speed HIGHER than its VMC, it is a "centerline thrust aircraft" for pratical purposes... such as a 727... But dont think that all airplanes with tail-mounted engines are "centerline thrust", I remember the little Lear 23/24 having a VMC of 93 KIAS... and a bastard to keep lined-up on the runway if an engine failed at that speed... took FULL RUDDER pedal + leg muscles...
xxx
Hope this helps you in putting some more light on the subject -  Smile
(s) Skipper




User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 12, posted (11 years 9 months 1 week 6 days 4 hours ago) and read 3744 times:

Regarding autofeather...this is/was used on some piston-engine airliners as well, DC6/7, CV340/440 come to mind.
The max takeoff weight with autofeather inop was reduced to a rather large extent, due to performance considerations.

Also, the British consider Vmca (critical engine) to be dependant on crosswind, ie: the critical engine on a four-engine turbofan powered airliner, would be the outboard engine on the upwind side. This dated from the old ARB certification requirements.


User currently offlineB747skipper From , joined Dec 1969, posts, RR:
Reply 13, posted (11 years 9 months 1 week 5 days 10 hours ago) and read 3729 times:

Correct about the ARB certification, 411A...
The British CAA approval raises the VmcG of a few knots to compensate for detrimental crosswinds during takeoff... good thing to remember since that is a more conservative number...
FAA certification numbers do not consider any crosswinds...
(s) Skipper


User currently offlineSllevin From United States of America, joined Jan 2002, 3376 posts, RR: 6
Reply 14, posted (11 years 9 months 1 week 5 days 1 hour ago) and read 3700 times:

AAR90:

Good explanation, especially separating Vmc and stall. So many people confuse the two. I learned to fly twins in the classic PA-23, an airplane which will motor around happily below Vmc and well above stall, just leaning left all the way.

My CFI made the deliberate point of doing an entire 180 below Vmc just to drive the point home.

Below Vmc operation is obviously bad, but not nearly in the same realm as stalls below Vmc!

Steve


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