VMCG and V1 on short fields

[Rover1974]

OK, newbie question here....

Since V1 can never be lower than VMCG, let me ask:

What do you do when taking off a really short runway?

Let's say VMCG is 100 knots (and it acts as V1 as well), but by the time you hit 100 knots (V1) on the take off roll, you wouldn't have enough room to stop the plane on the remaining runway.

You'd think in that case you'd simply lower V1 to account for the short runway... but the dilemna is that you can't do that because V1 cannot be set lower than VMCG. So what do you do?

What am I missing here? I know I'm missing something.

[NigelOnDraft]

Easy - you can't go....!

NoD

[timzsta]

If Vmcg is greater then V1 we cannot take off.
If V1 is less then Vmcg we cannot take off.
If V1 is greater then Vmcg then V1 is reduced to Vmcg.
V1 must never exceed Vr.

The above form the basis of several questions in the JAA Performance Examination.

You will be Vmcg "limited" on hot days, high mumidity, high temperature at light weights.

Hope that helps.
(edited for spelling!)

[Alex Whittingham]

Er..timszta..

Your first two statements are saying the same thing.

The third needs to be qualified so that you only reduce V1 to equal VMCG when you need to and when a range of decision speeds is available that allows you to.

Your fourth statement is correct.

VMCG is most limiting when the asymetric thrust is high, that is at low altitudes, low humidities and low temperatures. You are most likely to be VMCG limited at light weights because V1 tends to be greater when you're heavy.

[LEM]

One option would be to limit thrust (derate) to lower Vmcg.

[ferrydude]

How about a 3 engine ferry of a 4 holer?????

[quid]

On a 3 engine ferry, there is no V1. If you haven't rotated, you abort. Yes, you may very well go off the end. There is no such thing as "balanced field". We do, however, make some computations that are not used in line operations.

Wmc and W1 are calculated. (Yes, those are "W"s.) Where Wmc is a weight that assures min. control speeds, and W1 is a field length limiting weight. If W1 is less than Wmc, we can't go.

[ferrydude]

So how is VMCG different for an all engine versus 3 engine take off on a 4 holer?

[Keith.Williams.]

The original question implies that the aircraft is ASDA limited. This could be overcome by reducing the mass. This would not reduce Vmcg so V1 would remain at the stated value of 100 Kts.

But at the lower mass the aircraft will accelerate and decelerate more quickly. This will reduce the distance required to accelerate to V1 then stop (ASD). This might be enough to enable the take-off to be carried out.

[akerosid]

I remember a few years back, when EI had 747s (and since they've long stopped operating, I hope I'm not breaching any confidentiality!), the airline used to operate them from Dublin to SNN, en route to the US. The acft were usually pretty light for this sector and the fuel burn for the sector was usually around 6t.

HOWEVER, due to the light takeoff weight, the aircraft would face a situation where its V1 was less than its VMCG and consequently, its fuel load was bumped up to a standard 25t, to deal with this.

On a purely academic point (though probably not, if you're experiencing it), it's surely illogical for the VR to be below the VMCG, because you'd effectively be saying that the speed at which the aircraft can safely lift off is less than the minimum control speed on the ground. The logic of the V1 not being below VMCG is that you can't have a decision speed which is less than the minimum speed you need to control the aircraft?

[quid]

ferrydude-

"So how is VMCG different for an all engine versus 3 engine take off on a 4 holer?"

The numerical value is no different. But.....there is a big difference in "how" and "when" you get there.

I'd be happy to try to answer your questions, but I'm not sure what you're asking. ???

[EESDL]

Apart from the numerous aspects of getting airbourne with one donk out, just as important, it is the ability of the ac to stop in the distance remaining. Hence the figures of 3 out with 4 engines.

[mutt]

Looking at this another way, you have a VMCG of 100 kts, in order to use this speed in a balanced field calculation you require a minimum runway length of x,xxx feet. So if your V1 is less than VMCG you are left with the following options:

1: Increase V1=VMCG provided runway length = x.xxx feet.
2: Use a different runway.
3: Use a different thrust rating.
4: Wait for the temperature to change.

EESDL, accelerate stop distances dont account for the use of reverse thrust, therefore how will the planned stopping distance to affected by the number of engines which you lose?

Mutt.

[LEM]

On a wet rwy acc stop dist do account for reverse thrust for stopping, and a screen height of 15 ft instead of 35.
The above on a twin. Any differences on a 4 holer?

[NigelOnDraft]

akerosid

<<HOWEVER, due to the light takeoff weight, the aircraft would face a situation where its V1 was less than its VMCG and consequently, its fuel load was bumped up to a standard 25t, to deal with this. >>
I think you're dealing with a problem to do with your/EI's performance calcs.

In general you have 2 limiting V1s:
1. "Go V1" - the minimum speed at which the takeoff can safely proceed if you lose an engine at this speed. This is the one that must be >VMCG, or you will be off the side...
2. "Stop V1" - the maximum speed at which you can stop from - and this is the one which must be less than VEMBS (something like this!), or you'll run off the end with the remains of your brakes on fire.

For a given runway / consitions etc., there will be 1 weight where these 2 are coincident - the balanced field. At all lower weights, there will be a V1 spread. Its up to your Performance calcs which one is used - or more likely, something in the middle...

I suspect you were "given" the "Go V1", which ended up below VMCG. Your solution (add fuel) was obviously valid, but I bet the "Stop V1" was greater than Vr and you could have just gone without loading the extra fuel.... or just round up V1 to VMCG. Just the author of the performance manual did not give you these options!

NoD

[quid]

LEM-

>>On a wet rwy acc stop dist do account for reverse thrust for stopping, and a screen height of 15 ft instead of 35.
The above on a twin. Any differences on a 4 holer?<<

That's a recent change to the certification rules. (About 5-6? years ago.) All ac certified earlier than that, and those under SR422a and SR422b, like the DC-8, DC-9, 727 etc, can't take credit for reverse.

In the context of the original question, mutt is correct.

[Alex Whittingham]

The most recent change to JAR 25 has added in a paragraph that specifically allows reverse thrust to be used in wet runway ASDR calculations, but not in the dry runway case. JAR 25.109(f)(2) refers. I wonder why JARs are starting to diverge from FARs?

[timzsta]

Alex - good points about my post. I was still knackered from a 14 hour shift the previous day!

To correct:
If Vmcg is greater then V1 we dont take off.
On hot days, high humidity, high altitude take off, low weight we need to check what our actual Vmcg is. If V1 is greater, we reduce V1 to Vmcg.
V1 must never exceed VR.

In practical terms - on Sunday when it was 38 degrees just down the road in Kent from Southend Airport. If you were positioning a 757 that had just had a respray at Southend on to Gatwick you would be very wise to take a careful look at your performance figures - short runway, light weight, hot day, high humidity.

Not so tired now.

[quid]

timzsta-


"If V1 is greater, we reduce V1 to Vmcg."

Normally, V1 IS greater than Vmcg. When do you reduce it?
Assuming you're using "balanced field" V speeds, did you mean to say you reduce WEIGHT to cause V1 to reduce to Vmcg?

Lest we leave folks with the wrong impression, hotter temps and higher altitudes cause Vmcg to DECREASE. Lower density altitudes cause Vmcg to INCREASE.

Agreed?

[mutt]

If V1 is greater, we reduce V1 to Vmcg.

I always thought that the concept was to have V1 equal to or greater than VMCG. Why do you want to reduce it? :confused: :confused:

Mutt.

[Alex Whittingham]

Oh b***er! Sorry timstza, I misled you. I meant increase V1 to be equal to VMCG. I was thinking about VMBE, its the heat.

[alatriste]

I have just found same old material regarding this topic.
DC 9 AFM supplied charts to calculate " WEIGHT FOR MINIMUN RUNWAY LENGHT AND SPEED DETERMINATION". Depending on DA conditions it was a minimun ATOW for a balance TO.
When OAT or PA decrease, Vmcg increases, and therfore the minimun TOW is higher to comply with V1/Vmcg restictions.