question regarding take off

[1515Blue]

how is aircraft weight increased if Vmcg is limiting?

[john_tullamarine]

Not entirely sure that I understand your question but, presuming that you are concerned about a very light weight takeoff with a resulting min speed schedule which is too low, and for which the VR is still giving an unacceptable V1, then the situation is -

(a) min speeds for V1 will be limited by Vmcg

(b) max speed for V1 will be VR .. which in turn relates to V2

(c) to up the V1, we need to up the VR and V2

(d) the max VR/V2 will be limited by the available runway, obstacles, etc etc ...

(e) the simple solution is to look at the RTOW chart/table for the particular runway and increase the speeds (or weight, if you prefer) so that the speeds you choose correspond to a weight which is not more than the max weight in the tables. This should work for most aircraft unless there is some strange handling characteristic referred to in the OM.

One thing to keep in mind in the very low weight situation is that Vmcg (FAR rules) is based on nil wind. If you have a decent crosswind, it is advantageous to increase the V1 somewhat (in a similar manner to the above) to increase to the realworld Vmcg as opposed to the certification figure.

[Checkboard]

Some aircraft may increase their performance take-off weight by performing a reduced thrust take-off. Less thrust = less yaw for an engine failure = higher Vmcg.

[john_tullamarine]

? .. run that past us again, Checkers ?

[mutt]

J_T,

On a contaminated runway your takeoff weight can be limited by V1=VMCG considerations, you are prohibited from using assumed temperature thrust reductions in these cases, but if you have a lower thrust rating, you "may" increase the takeoff weight.

Obviously this wont happen in all cases, but will depend on runway length and the ratings selected.

Checkboard, did you mean lower VMCG?

Mutt.

[1515Blue]

umm...
i'm not sure i understand the concept of being able to increase take off weight by reducing take off thrust. by reducing take off thrust wouldn't we be increasing the take off roll? and by increasing the take off roll wouldn't that limit the allowable take off weight?isn't what we want a lower Vmcg not a higher one? please explain further... i don't understand it too well. thanks

[john_tullamarine]

Mutt,

All's well with your comment ... I was just having a playful dig at Checkers ....

[mutt]

1515Blue

If we were only looking at the ability of get off the runway the higher thrust rating would give the greater takeoff weight on a fixed runway length.

But we also have to look at aircraft controllability in the form of VMCG, if the higher rating requires a VMCG of say 135 kts, the aircraft weight must be reduced to a point where the aircraft can accelerate to that speed and abort. The actual takeoff weight must be the lower of these two weights.

If we then use a lower thrust, say 10%, the initial takeoff weight would be lower, but in this case we only have to achieve a VMCG of say 129 kts, so this weight may be higher than the VMCG limiting weight for 135 kts.

So the final result if that for a lower thrust rating you get a higher takeoff weight! This obviously depends on the runway length and surface condition.

Make sense? :)

Mutt

[1515Blue]

thanks for your reply mutt,
so Vmcg increases with increased takeoff thrust? i always thought Vmcg was dependent on weight. please explain some more.
thanks

[john_tullamarine]

1515Blue,

Bit confusing, isn't it ? ... Vmcg, like Vmca, depends mainly on thrust levels as it is a matter of balancing the moment due to asymmetric thrust against what can be achieved with the rudder.

The rudder moment is limited by airspeed and cg ... with the certification Vmcg being taken at max aft cg. .. if the moment due to increased thrust goes up, the only way to increase the rudder moment is to increase the airspeed ... hence Vmcg goes up.

Mutt is talking about different certificated levels of engine max thrust (hence derated rather than reduced thrust takeoffs .. quite different animals). The sorts of crossovers he refers to are a bit like looking at different flap settings and optimising the takeoff for the particular conditions on the day.

Where the weight confusion comes in probably is to do with the observation than Vmcg normally is limiting at lower weights where the desired V1 is lower than what would be the case for normal commercial weight takeoffs. In the same sort of way Vmca is mainly a problem at low weights where the Vmca requirement can become more critical than the Vs considerations in scheduling V2.

[mutt]

1515blue,

VMCG has nothing to do with weight, its pressure altitude, temperature and thrust rating and the resulting aerodynamic forces over the rudder.

The VMCG speeds that i used above are about right for a B747-400 using Maximum Thrust of the installed engine at sea level/ISA and then Derating that engine by 10%. Therefore the VMCG is REDUCED as the engine thrust rating is reduced.

Please note that I'm talking about FIXED DERATES, although we are allowed to do Assumed Temperature thrust reductions, we always base the VMCG on the Actual Temperature.

Now if you really want to get confused, ask J_T about crosswinds and VMCG.... :)

Mutt.

[mcdhu]

J-T. I've only ever flown flex/reduced thrust t/o ac. Could you please give us a quick burst on the difference - in simplish terms? Thank you.
Cheers,
mcdhu

[john_tullamarine]

Mutt is better placed to talk on this one as he routinely works with derate situations on shiny aeroplanes .. while I only get to play with dinosaurs ....

I think of it along these lines ... and if I have it not quite right, then I am sure Mutt will correct me ...

Many turbine engines are available with different rated thrust levels depending on where the fuel limits are set ... the same, or very nearly so, engine ... but different maximum thrust outputs ... and the certifications are built around the maximum output for the particular spec engine. Structurally much the same engine but, from a certification viewpoint, different engines.

Now turn the idea around ... if you have a big engine with lots of grunt, there is no reason why it cannot be operated to mimic an engine set to a lower maximum output. If this is certificated as an optional way to operate the engine then, in effect, it is a bit like being able to unbolt an engine of one thrust rating and bolt on another of a different rating. The certification data (ie AFM stuff for pilots) will differ depending on the rating adopted. The sorts of reasons why this might be desirable could include the Vmcg discussion here (as the certification Vmcg is a fixed speed depending principally on maximum thrust output), maintenance implications generally, and so on.

With an engine of a given rating, however, it is common to operate at reduced thrust (generally using the familiar Assumed Temperature approach), where practicable, to save on hot end maintenance costs. While we might do this quite routinely, the engine still remains able to operate at the rated thrust output and the certification (and AFM) data are based on that maximum level.

While I haven't any direct experience with derate operations using the same engine on the wing .... I guess that one could schedule both a derate, and a reduced thrust takeoff for that particular derate, at the same time ... Mutt will be able to comment further on this aspect of the matter ...

Hence two rather different animals when you look at the detail of the thing ...

[mutt]

Mcdhu,

Boeing also use the FLEX method of reducing thrust by assuming that the actual temperature is higher than it actually is, they call it the Assumed Temperature Method (ATM) of thrust reduction.

This method of reducing power comes with a number of restrictions, such as the requirement to demonstrate that the engine can provide maximum takeoff thrust, cant be used on contaminated runways or with MELS/CDL’s and are Limited to a maximum thrust reduction of 25% of the installed engine.

These restrictions severely penalized the amount of times that reduced thrust could be used so the concept of DERATED engines was initiated. This is basically treating the engine as if it’s a smaller completely different engine, complete with its own AFM data; this removes most of the above limitations and also allows for thrust reductions greater than the ATM limit of -25%.

Take the GE-90-90B engine which is rated at 90,000 lbs of thrust, this engine has two fixed DERATES picked by the airline, TO1 and TO2, we basically limit the installed engine thrust by –10% and –20% and treat this aircraft as if it has 3 different types of engine.

Approximately 90% of our takeoffs are at TO1, the vast majority of these are then combined with Assumed Temperature Method thrust reductions and an increased environmental envelope up to 69°C, so thrust reductions of approx 35% (-10% DERATE and then -25% ATM) are achieved and the takeoff is planned using approx 65% of the installed engine thrust.

The purpose behind all this is to increase engine life and decrease maintenance costs. It can also provide greater takeoff weights on contaminated runways due to the lower VMCG as discussed above.

There is also a “feeling” that a large engine working at less than full grunt is less likely to fail during the takeoff.

Cheers

Mutt.

[quid]

Hi mutt,

"VMCG has nothing to do with weight".......well that's not quite true in all cases. On my aircraft the Vmcg varies with weight 4-5 knots (as I recall). Not much, but it has to be accounted for.

We haven't talked in a while. Hope things are going well for you.

[john_tullamarine]

quid,

Nothing to stop the manufacturer scheduling multiple speeds.... might we enquire what aircraft you are considering ? ... might be interesting to have a look at the cg envelope ....

[mcdhu]

Thanks to you both, J_T and Mutt, for the excellent explanations; it must make for a very thick book of RTOTs!
Cheers,
mcdhu

[john_tullamarine]

... ah ... thank heavens for that ...... helps us to pay the rent ...

[mutt]

Quid,

How are you doing? I should have guessed that someone would pick me up on a statement like that :) I presume that you are talking about the DC8 with CFM engines? If I remember correctly that beast also came with multiple takeoff power settings for the inboard and outboard engines? So it really cant be classed as “normal”…. :):)

Cheers

Mutt.

[quid]

Mutt, JT,

I have 2 models of the venerable DC-8 (-61 and -63), both with JT3-3B or -7 power. But, my post last night was from memory (and I should know better than that at my age). <g>

The AFM (which I now have open on my desk) shows no change of Vmcg with weight, but there is approx. 4 knots difference in Vmca for each 60,000 lbs. change in weight.

Mutt - I'm still doing test flights, 3 engine ferries and my Designated Examiner flying. They keep throwing money at me, so I don't know when I'll retire. Life is good. Norm

[john_tullamarine]

.. ah, then .... that explains it ... we have always seen detail and quality from Long Beach ...

[mutt]

Quid,

Thanks for the correction, saves me from having to pull out our old DC8 manuals. I'm glad to see that you are putting off retirement, hang in there as long as it stays "fun".

Good Luck.

Mutt.

[1515Blue]

thanks for all the helpful answers! i do appreciate the time you guys have taken to explain to me Vmcg/Vmca. still digesting most of the stuff though ... thanks again!

[john_tullamarine]

... only one golden rule .. if you can .. stay away from them ...