PDA

View Full Version : Relationship between V1 and Vmcg?


moon11
4th Jul 2002, 22:17
Anybody knows answer?:)
What if V1=Vmcg:p
Thank you very much:eek:

FlapsOne
4th Jul 2002, 23:59
Then, I would suggest, V1=Vmcg!

Pegasus77
5th Jul 2002, 01:27
V1 is not allowed to be lower than Vmcg, wouldnt make sense anyway. On top of that, keeping an airplane on the runway at Vmcg with an engine failure is almost impossible as well, it is a certification-speed; I'm more at ease when the V1 is clearly above.

mutt
5th Jul 2002, 04:22
Moon11,

Ask yourself what are you trying to achieve by setting a V1 and what are you trying to achieve by using VMCG? What are you basing these speeds on?

Then ask yourself what would happen if V1 were 10 kts lower than VMCG. What sort of protection would you lose following an engine failure at V1?

I just think that if you are flying, its better for you to comprehend the relationship yourself rather than having someone like me tell you………

Cheers

Mutt :):)

john_tullamarine
5th Jul 2002, 04:23
The regulatory design and operating protocols permit V1 to be as low as Vmcg. Several problems arise which are not generally well known (which is why Mutt and I keep on talking about them)

(a) Vmcg is determined for nil wind in US practice. If there is much of a crosswind then the hazards start to increase dramatically in line with the increases in the real world Vmcg as compared to the certification value. It is, of course, not as simple as this as other factors are at play as well .... but the rapid onset of loss of control is very real and ought not to be downplayed

(b) be careful of rapid thrust changes as the dynamic handling characteristics might be different to what is expected ... this is more a concern with Vmca than Vmcg but could be a surprise factor in a flex thrust situation where the pilot elects to push up the levers aggressively .. far better to caress the levers ..

(c) the conventional runway width minima are based on geometry rather than handling problems. Under critical conditions, the pilot may well not be able to contain the aircraft within the normally accepted runway width .. ie one could end up in amongst the daisies on a width limited runway...


Sometimes the situation dictates a low V1 (eg short runway). A significant sucker bait problem can arise on a long runway at low weight (eg short ferry flight) where the uncritical pilot might well just accept the low speed weight-related schedule in circumstances (eg aft cg with a decent crosswind) where the intelligent selection of an available higher speed schedule would materially reduce handling risks during the takeoff ...

moon11
5th Jul 2002, 23:25
mutt,john, Thanks:)
I am reading at definition Vmcg:
Ground minimum control speed is the minimum airspeed at which the aircraft can lose an engine during takeoff roll with the remaining engines at takeoff thrust and can maintain directional control by use of full rudder deflection with NO
nosewheel steering.
As DC9 little pilot I didn't read anything about Vmcg-probably because of tail mounted engines.
If you lose engine before Vmcg-lets say at 100 Kts-can you use steering?
If you are too light on 747,you can use derate thrust, V1 will be higher,but still I dont see practicaly what that means

:confused:
Thanks in advance..
............................
Beer or not beer ,? is now?

john_tullamarine
6th Jul 2002, 06:39
The problem with definitions is that

(a) they can change over time as the subtleties of the design standards change

(b) there is often more of importance in the bits which the definition leaves out formally but which are built into the practical determination of whatever is under discussion during the certification exercise... it is this latter concern which makes fairly strict adherence to the manufacturer's AFM/FCOM procedures very important ... often there are comparatively critical things not stated specifically and which, as a result, the pilot cannot reasonably know about ...


The DC9 has a comparatively low Vmcg and, typically, DC9 pilots describe that Type as running on rails in the event of an engine failure ... don't believe it without some caution ... in the case of a very low V1 failure, the Diesel will skate like a beach ball on the waves in a strong wind ... I have some interesting runway flight test videos which demonstrate that characteristic quite convincingly.

If you lose an engine below Vmcg ? .. you close the throttles !! ... or else you run a very enhanced risk of departing the side of the runway and ending up in amongst the daisies ... again, the question is not answered simply as there are a number of things to take into consideration .. but the principle is set in concrete .. you stop !!

Mutt is better placed to speak to the 747 .. in general, with a derate takeoff (as opposed to using flex, or reduced, thrust), the V1 is able to be reduced as Vmcg is lower for the lower thrust output. Be mindful, though, that the selection of V1 for the typical FAR25 machine is not a simple matter and involves a number of considerations.

Ignition Override
7th Jul 2002, 05:31
John_tullamarine: Maybe some airlines allow pilots on a very light weight ferry flight to choose higher speeds for takeoff, but our company says that for a given weight and either flaps 15 or 5, we flip the speedbook pages to whatever is closest to our actual takeoff weight. As noted elsewhere on this topic, a takeoff with a very gusty crosswind can certainly yank the plane away from the centerline-even with two good engines, especially on a shorter series for a given type (DC-9 dash 10)!

Is this situation, if your suggestion is a safer method for keeping V1 speeds on an empty airplane at a safe margin above Vmcg, then is our normal procedure dictated by the original Douglas Flight Manual or strictly by some company or FAA legal requirement, in order to be consistent and standardized throughout the various types here?

Several years ago I asked one of our company Check Airmen whether he knew of any information about actual Vmcgs for a given weight, and he said that he had not seen such info. Maybe it is hidden in a file by the various Fleet Program Managers or such Chief Pilots?

'%MAC'
7th Jul 2002, 06:14
If I understand what you’re proposing, Eastern used this technique for take-off into potential wind shear situations. The idea is to determine the longest suitable runway and use the max weight speeds for the min flap setting that the particular runway will accommodate. So if light, you’re using flap 5 with the higher speeds of a heavier weight along with the longest (logical) runway available.

I've used this technique flying out of DIA, ORD, and smaller hubs; current company does not have a policy on this matter. As long as I don’t exceed the tire speeds and have 35' by the end it seems sensible.

john_tullamarine
7th Jul 2002, 08:55
Ignition Override,

I can only suggest that you discuss the matter with your flight standards people and see why they adopted that policy. If company procedures specify the adoption of a strict weight based speed schedule then I would go down the path of trying to get the policy relaxed somewhat.

The certification ignores crosswind for Vmcg determination and the DC9 procedures, like most aircraft, reflect the certification procedures ... it is expected that the operator use this as a basis for their own procedures.

Now, you are perfectly entitled to go at minV1 in a strong crosswind, have a failure, and run off the runway ... all I am suggesting is that, by using speeds appropriate to a higher weight (but not exceeding the RTOW for the runway in the conditions) you avoid an easily avoidable problem ... your choice (or, more particularly, your company's choice) ... but I know what my preference is ...

Having seen the real world dynamics for the diesel at low V1 with a strong crosswind in a failure situation .... it is a matter well worth considering.... why stick your neck out and accept a potentially very hazardous situation which is largely avoidable ?

Normally you won't get any additional information other than by cultivating the design and flight test organisations .. Douglas used to run some pretty good operator conferences and the delegates would get lots of good gen ... however, it is probable that your -10 will have a real world Vmcg increase somewhere in the order of one half of the crosswind .... Vmcg is not really related to weight .. except to the extent that, at lower weights, as the V1 reduces in the typical BFL related data, the Vmcg consideration eventually limits V1 reduction.

%MAC has the picture it would appear ...

.. and we should keep in mind that, in the OEI case, the climb performance improves at speeds a little in excess of V2min anyway ...