Engine failure below Vmcg

[bookworm]

FAR 25.149(e) specifies in some detail (e.g. maximum 30 ft lateral deviation) the controllability in the effect of an engine failure at or above Vmcg.

What certification "promises" are made regarding control in the event of an engine failure on the ground (just) below Vmcg? For example, is there a requirement that nosewheel steering is sufficient to maintain the centreline? ... at least keep me the right way up?

What differences for a subpart 23 aircraft?

[411A]

What differences for a subpart 23 aircraft?

14CFR23 aircraft don't have any Vmcg issues because none are specified...these aircraft will happily go in the weeds at nearly any speed.

[Back Seat Driver]

Minimum Control Speed on the Ground: VMCG
JAR 25.149 Subpart B FAR 25.149 Subpart B / JAR/FAR 25.149
VMCG, the minimum control speed on the ground, is the calibrated airspeed during the take-off run, at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the aeroplane with the use of the primary aerodynamic controls alone (without the use of nose-wheel steering) to enable the take-off to be safely continued using normal piloting skill.
In the determination of VMCG, assuming that the path of the aeroplane accelerating with all engines operating is along the centreline of the runway, its path from the point at which the critical engine is made inoperative to the point at which recovery to a direction parallel to the centreline is completed, may not deviate more than 30 ft laterally from the centreline at any point.”
VMCG must be established, with:
The aeroplane in each take-off configuration or, at the option of the applicant, in the most critical take-off configuration;
Maximum available take-off power or thrust on the operating engines;
The most unfavourable centre of gravity;
The aeroplane trimmed for take-off; and
The most unfavourable weight in the range of take-off weights.”

Below Vmcg, I guess you're a test pilot and do what ever it takes, including Nosewheel steering usage.

[safetypee]

With an engine failure below Vmcg – i.e. below V1, you should reject the take off. This involves reducing thrust to idle, which removes the lateral thrust asymmetry and extreme control requirement. The aircraft can be returned to the runway centreline with normal control use – rudder is still effective and NWS can be used at the lower speeds.

[lomapaseo]

With an engine failure below Vmcg – i.e. below V1, you should reject the take off. This involves reducing thrust to idle, which removes the lateral thrust asymmetry and extreme control requirement. The aircraft can be returned to the runway centreline with normal control use – rudder is still effective and NWS can be used at the lower speeds.

Agree

Of course if the engine failure happens to be a stuck fuel control, the weeds should assist you in stopping.

[john_tullamarine]

This thread highlights one of the problems with understanding what certification is about.

There are no guarantees, only measures of probability (read "very educated finger in the wind guesses") and repeatability (read "it's not much use if the data used is too airy-fairy").

Vmcg is a very artificial concept intended to provide the low speed line-in-the-sand for takeoff sums. Whether you can or cannot maintain control a bit below (or above, for that matter) (the certification) Vmcg on the ASI depends on a bunch of things which relate to how close you are to replicating the certification conditions .. thrust (flex, elevation, OAT), CG and crosswind, especially.

NWS, for instance, will be influenced greatly by CG .. the certification approach ignores it on the basis that, for the aft CG case, it's probably not going to be of much use in any case.

For the line operation situation .. if you are faced with a speed schedule on the Vmcg (or Vmca) limit, then consider the factors which go into the certification numbers and how your actual situation/configuration might be positioned .. but be prepared for the need for rapid decision making .. in the case of failure on the ground, you might well be in the situation of being near-instantly out of control ... of course, you snap the throttles closed and hope that the thrust follows .. otherwise it is time for dodgem car antics. If you have a failure in the Vmca limiting case, say, early in the rotation .. you may need very aggressive aileron control, according to Type characteristics.

And, if we sit back in the smoking chair and contemplate the matter over a wine .. why are we launching at minimum speeds ? If the runway is longer than needed and it is just a case of low weight .. why not adopt an appropriate higher speed schedule and avoid most of the worry ?

If everything is loaded up against you .. and you have to go (ie can't delay or do something else to reduce the risk) .. that's fine .. but one would feel a bit of a goose crawling out of the wreckage knowing that one really could have waited half an hour for the wind to die down a bit before launching ...

[SNS3Guppy]

And, if we sit back in the smoking chair and contemplate the matter over a wine .. why are we launching at minimum speeds ? If the runway is longer than needed and it is just a case of low weight .. why not adopt an appropriate higher speed schedule and avoid most of the worry ?

A valid question indeed. Backstory may be helpful. This thread was spawned from the thread cited below, from the Private Pilot forum. I think reference to that thread may prove illuminating.

http://www.pprune.org/forums/showthread.php?t=325406

The original poster would appear to be asking about Vmc, rather than Vmcg, but goes on to postulate 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.

[alwayzinit]

Engine Failure below Vmcg.

In a previous life we used to operate B757-200s into Skiathos, 5317ft long 30m wide, if memory serves!

A discussion arose over the pros and cons of full pwr t/o as stated in the books on rwys 6000ft and less and using derated thrust(not authorised). The concern being control below Vmcg.

The point being if you have a catostrophic failure below Vmcg the stopping before the end of the hard top is not the problem, but going off the side is.

So in various sim run throughs the big thing was to get rid of the assymetrical thrust asap, not worry about reverse thrust(for obvious reasons) and concentrate on brakes and nosewheel steering.

To stay on the runway rapid and aggressive inputs were the order of the day, not ideal but workable.

In the end the argument about "bad rated pwr" over "good derated pwr" was never really settled conclusively, but the results by the guys authorised to fly into Skiathos did surprise the sim guys as not one went anywhere near the edge even in the most critical scenario.

Suffice to say the books never changed and the next season we stopped going there!

Phew!

[bookworm]

And, if we sit back in the smoking chair and contemplate the matter over a wine .. why are we launching at minimum speeds ? If the runway is longer than needed and it is just a case of low weight .. why not adopt an appropriate higher speed schedule and avoid most of the worry ?

"Launching" at minimum speeds? For once, John, I find your logic rather difficult to follow.

All take-offs start at a speed at or close to zero. We'd like most of them to end at a sensible flying speed as we climb away. Unless you have invented an alternative to Newtonian mechanics, that means that at some stage we have to go through all the speeds in between.

I think I understand what the certification "promises" are if an engine fails above Vmca in the air. I also understand from FAR 25.149(e) what the certification "promises" are if an engine fails above Vmcg on the ground. And yes, such certification "promises" are based on ideal conditions that are not necessarily replicated in the real world and are probabilistic, not certain -- that doesn't make them entirely worthless from a risk management point of view, just subject to the usual disclaimers.

But I don't know what, if any, controllability requirements follow an engine failure below Vmcg on the ground. There might, for example, be a requirement that control can be maintained if the power on the operating engine is reduced within a specified timescale, or that nosewheel steering must be sufficient to maintain the centreline. Or there might be no such requirement at all -- that seems to be the implication of your post.

Understanding what is "promised" under what circumstances by FAR subparts 23 and 25 provides a useful background for understanding the logic behind operational procedures.

[john_tullamarine]

To stay on the runway rapid and aggressive inputs were the order of the day, not ideal but workable.

I would have thought extremely appropriate .. even ideal .. and, hopefully, sufficiently workable for Government work.

"Launching" at minimum speeds?

slack wordiology strikes again, I fear .. how about if I correct the phrase to "minimum speed schedule" .. ie Vmcg/Vmca-limited V1/VR/V2 ?

just subject to the usual disclaimers

problems arise when many pilot folk overlook the disclaimers

.. controllability requirements follow an engine failure below Vmcg

first, and very important point, is that Vmcg (and Vmca, for that matter) are artificial certification animals which have no precisely direct application to line operations (other than in their invocation as limits in the takeoff speed schedules). What is more relevant is the real world value for the day .. for which the pilot doesn't have any data.

One thing is for sure, if some of the noise stops below too low a speed (this real world figure) then things will happen quickly and unpleasantly. There are no requirements below these line-in-the-sand values .. the aircraft probably will become quite instantly uncontrollable .. the only rational pilot response is to get rid of the thrust and stop/land/crashland.

Indeed, for most pilots who have had no exposure to such regions of the envelope, even a failure at a modest margin above the real world figure will rapidly become a departure.

Any implication that there is any future in continuing a takeoff in such circumstances was not intended in my post and is quite incompatible with real world physics.

I don't see any "promises" in either Part 23 or Part 25 ... rather I prefer to observe that there are high levels of probability of successful outcomes in some emergency circumstances in Part 25 and very, very much lower levels of probability in (many) Part 23 birds .. I recall several colleagues who held different views ... and are no longer with us.

Particularly with smaller aircraft, the certification kingdom has branched into so many subparts that generalised discussion is fraught with confusion for those who don't play in the certification sandpit.

If I am launching in a small twin under reasonably limiting conditions (weight/Hp/OAT) .. and one quits below blue line .. then I have a high confidence that my preferred option will be to miss the big trees during the subsequent crash landing .. far too many people have demonstrated the unsuccessful mode of continuing a takeoff in a light twin with a failure at low speed .. not to say that it can't be done in some appropriate circumstances .. but I would want to be VERY proficient in the particular ship AND be launching out of a very benign aerodrome when considering the surrounding terrain/cultural obstructions.

[bookworm]

Thank you John.

I don't see any "promises" in either Part 23 or Part 25 ...

True enough. Slack wordiology on my part.

Vmcg (and Vmca, for that matter) are artificial certification animals...

I think the phrasing of my question and the title distracted from the real issue -- sorry. What I want to know is: "What certification requirements are relevant to the control of an aircraft on the runway after a critical engine failure?". If I had asked that question without qualification, someone would simply have quoted FAR 25.149(e) at me (actually, someone did anyway ). That's the only reason I mentioned Vmcg.

The consensus seems to be that with the exception of FAR 25.149(e), there are no relevant requirements, hence the need for a rapid reaction on the power or the very real threat of a departure.

If I am launching in a small twin under reasonably limiting conditions (weight/Hp/OAT) .. and one quits below blue line .. then I have a high confidence that my preferred option will be to miss the big trees during the subsequent crash landing ..

And I'm with you 100%. The point that I make on the thread SNS3Guppy cites is that it's that blue line speed that marks the threshold between an instant reaction of chopping the power and the possibility of taking the aircraft into the air. Vmca is not the threshold, nor is Vr.

[Back Seat Driver]

Bookworm, apologies for quoting scripture to what you admit, was a poorly worded question. BUT, to contend that 'blue line speed' is the defining speed between aborting and continuing a take-off, is simplistic in the extreme. Maybe in your world that makes sense. I would be interested to know how you account for a balanced field length performance calculation involving a critical engine failure at V1 and before attaining best single engine climb speed. More often than not on heavy Quads. the TOSS is several thousand feet further down the runway past V1. How would that affect your stop margin? What would your screen height be at the end of the runway. What clearance would you have with obstacles on your take-off path. If you are unfamiliar with heavy aircraft on wet runways you probably won't see my point.
Lastly Vmca is a very valid consideration with an engine failure at V1 in a lightly loaded A/C using flex thrust for take-off, ie. whether to select TOGA thrust or not.
I think I'll stick to the wisdom of the many performance engineers who have investigated the 'why's and wherefore's' of A/C performance rather than settle for the 'Blue Line' panacea you subscribe to.
If as I suspect you are only interested in light twins etc. then please ignore my rant, because I would be surprised if any heavy metal drivers would be swayed by your argument.

[SNS3Guppy]

And I'm with you 100%. The point that I make on the thread SNS3Guppy cites is that it's that blue line speed that marks the threshold between an instant reaction of chopping the power and the possibility of taking the aircraft into the air. Vmca is not the threshold, nor is Vr.

Blueline, or Vyse, is a little over the top or overly simplistic for a rejection decision in a light twin. We're talking about a light twin (that is, a light Part 23 twin here...specifically from the other thread a Cessna 340). Waiting to rotate or make the decision with regard to going to flying until achieving Vyse is ridiculous, and additionally puts you at considerable risk in the event of a high speed rejected takeoff.

You've asked several times here, and the other thread, what the difference is between losing your engine on the ground and in flight, and do not seem to differentiate between the two. There exists a period during the flight when one is better losing it on the ground, and there exists the majority of a flight when one is better losing it, and handling it, in flight.

Among the issues with waiting on the ground until the "blueline" for the engine failure are a lack of published data for your decision. What manufacturer suggests or provides performance data for remaining on the ground until that point? None. Each provides a rotation speed along with the relevant data, and each counts on you accelerating to the blue line reference once airborne. Additionally, attempting to come to a rest on the remaining runway from that speed with no reverse, inadequate brakes, high energy, admittadly poor high speed ground control or steering, etc is not in your favor.

If you happen to be in a light twin that isn't capable of sustained flight on one engine, as some are, then you are correct in surmising that you will be ending the flight in the near future. That's just something you have to account for before you ever leave the ground, to include forced landing sites, clearways, obstacle clearance, etc...before you ever start and engine. You're in the same boat as a single engine airplane with respect to the fact that you'll be landing...but not at all with respect to the way you do it.

You do NOT want to do the takeoff below Vmc, nor do you want to wait until Vyse to get off the ground. If you are in the air and lose an engine while below Vyse, that's not automatically cause to shut down the show,either. Part of your multi engine training in a light twin, if you had any, should have been accelerating to blue line or Vyse as you clean up. The major difference in the way this is done with a light twin vs. a Part 25 airplane is that you can't wait for 400' or any other "safe altitude" because you may not climb if you don't get it cleaned up.

What about Vxse? That speed as best angle, will be less than best rate...if you're above Vxse and not yet to Vyse, are you still going to use Vyse as your refusal point...in the air? You're above a flying target speed, but will still use "blueline" as your point to shut down the good engine? That makes no sense.

The point is that it's far too simplistic to suggest that simply because the airplane may not climb out on two engines, there's no difference in when one takes off or elects to reject, and it's far too simplistic to suggest that one wait until "blue line" speed to consider rejecting the takeoff. Moreover, it's a rather ridiculous assumption to put your decision speed at some point well after the takeoff.

Use your published data. It will provide a rotation speed. That's where you rotate. You're best rotating at that point, climbing, cleaning up, accelerating to your intended climb speed, and going flying. If you have a problem at some point in that evolution, then handle it according to the manufacturer's procedures (fly the airplane, clean up, go fly). If directional control becomes a problem and you run out of aerodynamic control and have no means of increasing airspeed, then retarding power on the good engine is your only choice. In many cases, you need not retard it very far; a slight reduction may be acceptable as you seek a higher speed, lowering the nose and accelerating, before restoring the power. It really is possible on some light twins to lose the engine, accelerate to Vxse or Vyse and climb while you bank into the good engine, clean up, and return to land.

[bookworm]

Back Seat Driver

I'm sorry if you found my aside offensive. I can see you did more than simply quote the FARs.

If as I suspect you are only interested in light twins etc. then please ignore my rant, because I would be surprised if any heavy metal drivers would be swayed by your argument.

I'm interested in the procedures and best practices for both subpart 25 and subpart 23 aircraft, and in particular understanding the differences between them. The comment about blue line speed was very much in the context of subpart 23 aircraft, which I thought was John's implication in using the phrase "small twin". I have no wish to sway anyone with an argument that procedures for a subpart 25 aircraft should be any different to the well established ones used by operators of such aircraft everywhere, only that there are reasons that the procedures might be different for a subpart 23 aircraft with no performance guarantees.

[bookworm]

Use your published data. It will provide a rotation speed. That's where you rotate.

I couldn't agree more in principle, though you might want to consider how that sits with you apparent assertion that rotation below Vmca is unwise. Here's the published data for a PA30B Twin Comanche from the Piper Aircraft Corporation POH:

Vmc 90 mph
Vy 112 mph
Vx 90 mph
Vyse 105 mph
Vxse is not published but with a wopping 260 fpm OEO at Vyse, I doubt it's going to be much less than 105 mph.

Performance data is published for a lift off speed of 80 mph attaining 91 mph at 50 ft.

Here's the manufacturer's description of a short field take-off:

"If the airplane is off the runway at less than 90 mph, it is essential to fly level a few feet off the runway immediately after lift-off until reaching 90 mph... Since the airplane cannot be controlled in the event of the sudden power loss in one engine, be ready to reduce power promptly."

It really is possible on some light twins to lose the engine, accelerate to Vxse or Vyse and climb while you bank into the good engine, clean up, and return to land.

On some, yes, I'm sure it is. On others, like the PA30B, I think I'll stick to committing at Vyse thanks very much.

[DFC]

Bookworm,

Any twin engine aircraft engine failure below VMCG the immediate reactions are close throttles, maintain directional control and stop ahead.

The same applies to a failure below VMCA

The twin comm's performance category means that it is likely that a forced landing will be required with an engine failure shortly after take-off. With that in mind, there is no advantage or loss in permitting the aircraft to become airbourne before in this case VMCA.

In fact provided that the immediate reaction to an engine failure is to close both throttles then it is just as safe to become airbourne below VMCA and accelerate in the climb to 50ft - the take-off distance figures would allow for this, than to try and keep the aircraft on the ground resulting in a wheelbarrowing situation.

Ever seem a twin comm have an engine failure below VMCA with just the nosewheel firmly on the ground? Not something I would like to try.

If it is a long runway and you are airborne in something like a fully laden senneca 1 and at Blue line speed, I would keep the gear down until there is no longer a posibility of landing back on the runway because if one engine fails, I am going to land back on as long as runway is available.

Thus as has been said earlier, light twins are esentially no better than singles until cleaned up, well established in the climb and of course in some cases at less than MTOW.

Just to make it interesting you should note that there are aircraft where VMCA is less than VMCG.

In FAR-25 aircraft, being performance A category V1 must be a factor above VMCA / VMCG. This requirement ensures controlability in the event of a stop as well as a go decision.

Up to V1 - stop. After V1 - go.

As has been said earlier also, the VMCA / VMCG figures are simply baselines that determine the operational speeds.

To put it in simple terms - your rotate speed in a C172 is a factor of the stall speed but you don't particuluarly note the passing of the stall speed during the take-off roll - you simply concentrate on acheiving the rotate speed and safely departing.

Regards,

DFC

[john_tullamarine]

to contend that 'blue line speed' is the defining speed

Yes, it is simplistic (and that is one of the attractions) in the same way that "reject below V1"/"continue above V1" is simplistic. Clearly there will be quite some variation in the application of such a decision process according to the Type .. but one needs to start somewhere in managing an engine failure without too great a need for contemplation at the time of the event.

As an aside, from my own history with light twin IFR renewals, I always used precisely this concept without ever having an examiner query my logic .. but then they all knew that, if they pulled an engine earlier, ... the other would follow by my hand .. this was made easier by the fact that I rarely was faced with taking a twin out of a short strip.

For a short strip departure, however, my philosophy would be altered to suit what I assessed to be a reasonable middle ground of risk management. Certainly for typical shorter field twin operations, if the operation is to occur, one has to lift off earlier than blue line and just accept a higher risk period until the aircraft is cleaned up and climbing away at a suitable speed. In general the greatest risk was driving out to the airport so it all needs to be kept in perspective.

The reality with light twins is that the takeoff is divided into three reasonably distinct phases .. low speed where one has no choice but to stop as best as one can arrange .. high speed where one may/should continue subject to OEI climb capability and surrounding terrain (and, perhaps, weather) on the day .. and a very BIG grey area in the middle which could take up a lot of time in discussion over a wine or three ...

More often than not on heavy Quads. the TOSS is several thousand feet

We all need to keep firmly in mind the reality that "light" and "heavy" (aircraft) are related in a similar fashion to "black" and "white".

The FAR 25 animal is characterised by comparatively high levels of redundancy and safety (ie low risk) while the simplest FAR 23 version is at the other end of the spectrum .. all very reasonable when one considers the target market, usage, and cost/benefit aspects, etc.

Point is one needs to be very careful when trying to compare apples to oranges .. trying to overlay Part 25 philosophy onto the Part 23 aircraft is fraught with difficulty.

[Back Seat Driver]

Bookworm, I agree with you in regard that you do what you have to do, on the equipment you're flying on the day. My only concern for you is that you realise that a PA30B is a single engined aeroplane with half an engine on each wing, and losing an engine on take-off will most probably result in the live engine delivering you to the scene of the accident. Otherwise it is a beautiful machine. All the best with your flying.
regards
BSD
John, Wise words. Casting my aged brain back to the days of light twins, I remember that just before pushing the 'loud levers' forward I would choose a decision speed (V1 in effect) based on all the variables you mentioned. Only on a long hard black top would that speed be "blue line".
Horses for courses, not hard and fast rules applied for every occasion.
ps. You don't always reject below V1, (many operators reduce V1 by 10 knots or so on a wet runway. ie. artificial V1) and Latitude to reject above V1 if sufficient Rwy remains and aircraft is not in a flyable condition.
-Interesting thread-

[SNS3Guppy]

With the introduction of the PA-30, you've now included CAR 3 into the certification mix, per the Type Certificate Data Sheet.

I've spent a considerably amount of time flying light twins from rough fields, dirt airstrips, one-way box canyons and the like, and have some appreciation for the requirements to operate them safely.

An airplane such as the PA-30 isn't really a single with two half engines when it comes to losing an engine. Nearly 80% of the available effective thrust is gone when dealing with the assymetric thrust condition. Whereas climb performance is determined by excess thrust, there may very well be little enough to leave no climb at all. What there is, however, determines how fast you drift back down, and subsequently, the point on the earth to which you drift. A very big consideration in getting back down.

We had a PA-30 for a time. I didn't much care for it, but it did the job. Much of my career in flight has been spent at very low altitudes, and that included much of my early twin flying. Climbing to a "safe" altitude was never an issue because we'd never be climbing there...even when I got to four engine airplanes, we seldom climbed above 500', operationally. Accordingly, not having the luxury of time and altitude, things had to be done right...which always included for having the necessary performance, planning our relationship to terrain well in advance, and always leaving an "out."

I submit that this is good practice no matter where, or what you fly; it worked for me then, it worked for me now. Trying two eek out of the shortest field you can, below Vmc, may be something you get away with for a time. It's not something you'll get away with for all time, nor is it prudent.

The specific certification numbers, as previously discussed, do not necessarily equate to the real world numbers, but one must remember that while the published minimum control speeds may be decreased by density altitude (normally aspirated light twin), reduced thrust takeoffs, different weights and forward CG's, the real world counterpart to those numbers may also be higher than published due to crosswinds, runway condition, cold temperatures, pilot reaction, a tired, well-loved airplane, etc.

Being conservative is a professional act that serves the interest of the airplane, the passengers or cargo, and certainly the crew.

[john_tullamarine]

Being conservative is a professional act that serves the interest of the airplane, the passengers or cargo, and certainly the crew.

Hopefully the majority would echo such thoughts ...