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underread east
7th Jun 2007, 11:53
We regularly fly our 752s at very low weight (where they are definitely not very happy). I have had various discusions with other crew about Vref30+5 as normal ops final speed (Vref 30 as low as 106KIAS), which is as per co and Boeing SOPs and 111kts in this scenario.
I am aware of Vmca restrictions, which of course should be more than catered for by 1.3Vs of Vref30, but have had several of my colleagues insisting that they WILL NOT go below 117 due Vmcg considerations. As I understand it, this is for the full thrust, MTOW situation of losing the critical engine at V1. By Boeing's own definition, 111kts should be perfectly safe in the event of landing, and for a G/A you must be above Vmca, otherwise they would not certify at this weight?
Your thoughts, please...

Dogma
7th Jun 2007, 12:52
On the B757-200 there is no requirement to check VMCA, the certification process identified that the you could control the aircraft throughout the entire low speed range.

You only need to check VMCG because you have no benefit from the 5 Deg of bank allowed in the VMCA case.

Any one else have a different view?

BOAC
7th Jun 2007, 13:50
1) Vmcg is not weight dependent (standing by to be shot down by a test pilot/perf engineer:))
2) Vmcg is irrelevant during the landing phase since you should never be at full thrust on the ground.

underread east
7th Jun 2007, 14:58
The irrelevance of Vmcg has been exactly my point.

In a land and stop situation you are at worst at idle trust, and best at max rev - plus with all engines, credit is only given for 1 rev anyway. ie full assymetric forward thrust, the worst gnd scenario for controllability, is not applicable.

In a G/A situation, the thing is still flying, and there again I see the irrelevance.

OR is the consideration of Vmcg to be for the G/A from a position of having already landed? Not an option if REV already pulled. Or from a low level, having touched down in the G/A, you are, as far as I can see it, still in flight.

UE

mustangsally
7th Jun 2007, 15:08
"Not very happy," at light weights. How is an airframe "happy or unhappy?"

I think what you maybe referring, is which airframe weight is easier of get a better landing from. Simple, airframes near max landing weight have greater stored energy to consum during the landing event, tending to be more stable from the operators view point. Light aircraft have less energy when compared to the heavier event. Now add some wind, some surface condition, and a tired operator.

I've seen a lot of varied operations. "I'll add x knot cause we are light, or cause I don't think the winds are this or that... " This adds energy back into the airframe. Then the great operator floats down the runway and comments, "See I was right the..... were incorrect."

Large aircraft have significantly varied landing weights. For example, the zero fuel weight plus say 10,000 kg of fuel may give you a landing weight of 175,000 kg, while the max landing weight maybe almost 130,000 kg greater. While your narrow body difference maybe only 25 or 30,000 kg. It all comes back to energy management.

Point, counter point.

underread east
7th Jun 2007, 15:15
Thank you, I understand the effect of less inertia, and the 752 is more difficult to fly at the bottom end of its envelope ie I have to work a bit harder, which I am not always averse to, despite what some say! But my question is about the argument for the consideration of Vmcg (117ish in the case of the 752) when approach speeds below this.

mustangsally
7th Jun 2007, 17:09
I think the gentleman with the suggestion that there is a difference between Vmcg and Vmca has the winner. With Vmca some bank angle is allowed were as Vmcg it almost all rudder.

Also take into account that with a Vref of 117 the actual touch down would be less, say about 112 kts and this in short order becomes will below Vmcg. In addition Vmcg is the sudden loss of thrust and then being able to control, recognize the situation and making the correct reaction.

Yee old DC-9-10 was a handfull at Vmcg.

bleeds off
7th Jun 2007, 17:49
Correct me if I'm wrong.

It seems to me that the relevant speed that applies on the approach phase (with regards to lateral controlability) is defined as "VMCL".

On my current type (ATR) "Vref" calculation is based on the greater between "1.3Vs" and "VMCL". Within the range of very light weights, VMCL is greater than 1.3Vs.

My understanding is that the discrimination between VMCL and VMCA lies on the flaps configuration:

VMCL: Landing flaps configuration.
VMCA: T/O flaps configuration.
For both speeds determination, L/G is retracted.

Then I guess that VMCL secures the lateral controlability, should one engine fail during the particular phase of G/A thrust application whilst the flaps are still travelling from LDG position to G/A position.

The slight nuance here being the fact that VMCA applies for T/O, when flaps are already set at the required fixed position, whereas the flaps retraction from Landing setting to Go-around setting during G/A phase had to be particularly covered, hence the VMCL notion.

That's why I find more appropriate to consider VMCL as opposed to VMCA during approach.

As for VMCG, I am not sure I understand its relevance during the approach phase.

Dogma
8th Jun 2007, 01:01
There is a lot of Horse S:mad: on this post.

On the 757 there is no VMCA or VMCL issue at any speed or weight, provided you comply with the normal speed regime.

Henry VIII
8th Jun 2007, 17:42
you should never be at full thrust on the groundGA is allowed after touchdown, until reverse deployment.

underread east
8th Jun 2007, 18:29
Thank you all for your comments, but I am still none the wiser.

The discussion between myself and colleagues has always had them using the (irrelevant) Vmcg argument for not flying an approach at less than 117Kts, and I am yet to be convinced that it is unsafe to fly at a slower speed than this when the Vref30+5 approach speed that boeing says is safe at low weights they consider safe at an aerodynamically identical speed (Vref30+5) when the aircraft is heavier (in benign conditions, light wind, uncontaminated runway - talk of gusts etc are not part of this discussion).

Going back to what Vmcg takes into account - MTOW, Failure of critical engine at V1, no reversers. In the scenario I mentioned initially, MTOW is not relevant, we are almost at MIN weight. In the stop case we decellerate with or without REV, and are safe. In the G/A case, we apply full thrust, and whether or not we touch down in the process, we remain in flight, so Vmcg is not part of the equation here either...

I just wish to confirm by belief that approach speeds below Vmcg are safe. I have much to learn and may well be wrong in this belief. I just wish to operate as safely and professionally as I can, and as ever standby to be corrected!

Again I thank you for your learned comments...

Dogma
8th Jun 2007, 18:58
I give up, ask one of your training Captains. One would have hoped you would have covered this already and I hope you are not a Captain.

Good Luck

Miserlou
9th Jun 2007, 09:41
Underread,
I agree with your position for the reasons you give.

Vmcg is not relevant on landing until you reject the landing below this speed (and how often do we do that?) but you're still bowling down the runway at a good lick and a long way before you'd need to worry about the speed if it were a take-off situation.

That is to say, if you make a go around after touchdown you just don't use full thrust until passing Vmcg.

You logic is sound and the rules reflect that.

BOAC
9th Jun 2007, 10:32
I still cannot believe this is being discussed!

We are now considering:-

1) a low Vref approach
2) a need to abandon the landing once on the ground (be VERY careful about doing that!:eek:). The chances of being any where NEAR full thrust before Vmca becomes the applicable speed are, I suggest, nil. On base training=roller landings Vmcg IS a consideration. On a 'normal' line approach, not
3) now we are planning for an engine failure?:confused:

several of my colleagues must live in a state of extreme anxiety.:)

Cryten
10th Jun 2007, 08:44
Vmcg a consideration on landing? that's a new one.

pull the thrust leavers, problem solved;)