assumed temp

[alexban]

Hy
One question regarding assumed temperature and selected speeds for take off.
Untill now we used to do like this:
I.e : TOW 48T.....check assumed temp calculations and find out we can use max temp 50C -coresponding to a max TOW 53T. Then go with the temp on other tables ,and find out speeds for 48T,then correction for 50C ,add-- resulting final take off speeds.
Now ,there seems to be some other idea on our company:
48T-find out 50C,and take the speeds from the assumed table,even though this speeds are calculated for a max TOWOF 53T.
The differences can be high,especially at low weights:
V1-119kts for TOW 45ts and V1-135 or more for 53T (from memory )
What do you think?
Is it unsafe?Is it like doing improved climb ?
From what I remember the braking force is directly corespondant to the mass of the object:higher the mass ,higher the braking action.
So,for a low TOW-low V1,higher TOW ,higher V1.
Then ,I guess is not safe to use v1 coresponding to a higher TOW for a lower TOW.Even though inertia would be bigger for a higher TOW,the braking action will be higher.For lowr TOW,lower inertia but also lower braking force,so if you use higher than calculated v1-an invitation for overrun.Am I right?
Thks..
Brgds Alex

[BOAC]

Alex - I believe what you are ?moving to? is common practice.

If you think about it, the tables say you can go at 53T at an ambient of 50deg. Therefore if that is your 'assumed' then it will go and stop at 53T. You are now going to roll at 48T. Thus you will reach V1 (53T) more quickly, ie more runway than required to stop, and more runway left than required to achieve a safe take-off. V2 is higher than need be, so again safe. Yes, a bit like Improved Climb, but you are not actually getting any 'Improvement' in RTOW.

The only cloud on the horizon was once raised in my last company by a friend - who received NO reply!!! Suppose we do the above, but abort at V1(53T), and for whatever reason, brakes, slippery surface etc, overun the runway by, say, 40', and injure some pax and damage the a/c. Would the lawyers have a field day or not when we could have used a lower V1 and stopped on the runway?

[alexban]

BOAC-indeed ,I'll reach more quickly v1 (53t) ,but in case of reject-actual weight is 48T-lower braking force comparing to 53T-so maybe overrun. The v1 for 48T can be 10kts lower than v1(53T).
In case of improved climb we have calculations done,but just ot take v1 for max TOW for any lower TOW doesn't seem safe to me.Why all the calculations then,just do one for max TOW?
The assumed 50C guarantees you'll stop for a TOW of 53T if you use the V1 for 53T.If you're lighter than 53T shouldn't you use corresponding v1?

[BOAC]

lower braking force - sorry, Alex, I don't understand? Far less energy to dissipate, same friction force from tyres/brakes.The assumed 50C guarantees you'll stop for a TOW of 53T if you use the V1 for 53T - so if you use that V1 at 48T?

a) You reach it FURTHER from the 'nasty' end
b) You have a lighter a/c to stop from that speed.

[mustafagander]

I don't see a higher V1 as any problem - it's an energy consideration. Your tables say you can accelerate/stop at 53T, so it is obvious you can comfortably do so at 48T. Same thrust, lower mass = V1 in less distance from brakes release. Max brakes at V1 (53T) with lighter a/c = less energy to dissipate in the RTO = shorter stopping distance.

My concern would be Vr and V2. Both are a function of TOW. Still, I suppose it is in the right (higher) direction given that V2 is almost invariably (on the jets I know) on the backside of the drag curve.

My airline uses the V speeds for the prevailing mass and temp, and the thrust for the assumed temp.

[alexban]

As I remember from school days,the braking force =M*g*m(braking coeficient)
As you see it's directly proportional with the mass of the object.
This is the reason for the spoilers after landing,to spoil lift-resulting mass increase-resulting higher braking force.
At 48 T (i.e.) the braking force will be lower than at 53T ,so the reason for lower V1(48T) than V1(53T)
So,if you use V1(53T) for 48T -a risk to not be able to stop.

This is why i'm concerned about this assumed procedure( increase V1 for lower weights)
Of course,somewhere I may be wrong,I just don't figure out where.

[QNH1013]

From my understanding, the Assumed Temp method is to save engine life. The V speeds are directly for the aircrafts performance at its actual weight. Ok, I see the safe arguments of an earlier V1, but why would you be using higher VR and V2 speeds than normal? You increase your ground roll. And effectively will be degrading your initial climb profile. ie, best gradient climb profile is at V2+20 (for Actual weight) If you start of at an abnormally higher V2, then your V2+20 will be at a higher than normal speed, which will then not be your best climb gradient speed but instead reduce the obstacle clearance angle.

[Empty Cruise]

Alex - as always correct. But the required braking force will of course be lower, since the kinetic energy to be dissipated is lower (M x GS squared).

So unless e.g. aquaplaning is a question, the braking energy required is (M x g x mju) / (M x v squared), which can be reduced to (g x mju) / (v squared).

Therfore - AFAIK - the decrease in braking force is exactly counterbalanced by the reduction in kinetic energy.

However, since we are now dealing with the required braking force, we have made good our ability to dissipate the kinetic enegy of a 53 T aircraft decelerating from the V1 corresponding to 53 T. Our actual mass is only 48 T. So - to my limited knowledge - we are looking at the following formula:

Brake force available for 53 T
------------------------------ = >1,0
Brake force required for 48 T

I doubt that a lawyer will be able to disprove the (classical mechanistic) validity of Newtons laws of motion. BUT - and that is a very big but - if we are looking at a wet runway where aquaplaning is a possibility, there might very well be an issue. I have no idea if assumed temp. is allowed on a wet runway - only starting my 73' course on 4/4 :D Looking fwd to it - maybe somebody in here could educate me in the mean time :O ???

Brgds from
Empty

[Ziggy]

Alex,

A heavier plane doesn't give you more braking force available, but a heavier plane needs more braking force to be stopped.

So, (I honestly don't really remember from my schooldays) are you sure that the formula you give is not for calculating the REQUIRED braking force?

Is 'g' the decelleration rate?

So from the formula as I (mistakenly?) read it, a higher A/C weight 'M' will lead to a higher required braking force.

Or the other way around:
at a given available braking force (for example max manual braking) and at a fixed braking action 'm', then a lower weight 'M' will give you a higher decelleration rate 'g''.

Spoilers after landing, or at rejected take off, kill the lift, bring the weight of the A/C on the wheels, thus giving the wheels more grip i.e. increase braking action. You will then need less distance to stop or you can stop a heavier A/C in the same distance, or you will need less braking force to stop your plane in the same distance, thus reducing brake wear and brake energy build up in case of a short turnaround.

FYI, also in my company, independant of use uf assumed temperature or not, we always use speeds for the actual TOW.

Greetings, Ziggy

[Kakpipe Cosmonaut]

What a few people have talked about is required brake force. The actual braking force (with either RTO or max manual braking) is technically possible to stop the a/c @53T prior to v1 on the runway. Therefore it will also happen @ 48T.
We used to do our calculations like this but changed a few years back to v-speeds at actual weights.

[mustafagander]

Stopping the a/c (or any other vehicle for that matter) is a matter of dissipating kinetic energy. Note the "M" (mass) component in the equation. For all dry runways the braking force available way, way exceeds that required. Logic demands that if it is possible to stop a 53T a/c under a given set of conditions, it is also possible to stop a 48T a/c under the same conditions. Remember - you have more runway available to stop because you achieve V1 in a shorter distance.

Unlike most pilots, I have experienced a full blooded RTO from V1 on a B747 classic many years ago. Awesome stopping force!! Within 3 minutes the brakes (steel) had welded and there the big bird sat for many hours - there aren't 8 spare wheels and brakes for the B747 lying around in most ports. (Yes 8 - it's not an error. Boeing allows 2 diagonally per truck to move the a/c.)

[ftrplt]

Alex,

looking at the wrong side of the coin; the lower V1 at the lighter weight is because of the GO case.

At the lighter weight, you can continue from a lower V1 and still get airborne in the runway remaining. As weight increases, you need to be faster (on all engine acceleration) before the engine fails so you have less accel required (on one less engine) to reach Vr.

Same scenario when reducing the runway length available, the V1 goes UP because you have less runway from Vef to accelerate to Vr and get airborne

(this is not always the case but most common, short runways and heavy weights will be stop limited)

[FE Hoppy]

one prob can occur if you are very light weight at max assumed temp and you use the speeds for the temp. your a/c may want to get airborn before v1.

[Old Smokey]

alexban,

A very good post with excellent responses, and no-one going off at ridiculous tangents, I hope that this is not one of them.

What you are essentially arguing alex, is the relative merits of two different techniques, i.e. use of speeds for your Actual TOW (48T) or for that achievable at limiting conditions (50T at +50°C). Everything in aircraft performance is a compromise, lose a little here, for (hopefully) gain a little more somewhere else. There are benefits and merits to both techniques, both are safe, and both are legal provided that it is allowed by the AFM.

To add my tuppence worth to the discussion, and to address your questions regarding the use of higher V speeds -

(1) "Is it unsafe" - No, More safe for the continued Takeoff, Less safe for the rejected Takeoff, but still within normal parameters of safety. As several posters have alluded, if the aircraft can stop from the same V1 at 53T, it certainly can at 48T, and with ASDA in reserve. It is not, as you put it, "an invitation for overrun". For the continued Takeoff (i) Wind Shear protection is considerably enhanced at the higher V2 [On pre-reduced Thrust aircraft it was common practice to use the V speeds for the highest permissible TOW when wind shear was possible], and (ii) OEI climb gradient will be improved, see the next item for this.

(2) "Is it like doing Improved Climb?" - Yes, you are doing a 'de-facto' improved climb. V2min may be as low as 1.13 Vs, where 2nd segment climb is obviously acceptable, but well below the optimum available at speeds closer to Vmd. There is no upper limit prescribed for V2, but IN MY EXPERIENCE I have not encountered a useful 'Improved Climb' V2 above 1.38 Vs. To put it back into simple terms, if the aircraft can achieve the requisite OEI gradients at the V2 for 53T, it will certainly exceed them at 48T, even if, in extreme circumstances, that V2 for the actual weight was above Vmd (A pretty extreme case).

The summary of the above waffle is -

Accelerate Stop performance is degraded, but still within normal safety limits.
Continued Takeoff OEI performance is improved.
Wind Shear protection is improved.

Both techniques are acceptable and safe. Mutt, (a highly respected Performance contributor) I observe, specifies the use of Assumed Temperature for his company, but then uses FMC V speeds for Takeoff, that is, V speeds for actual weight. Some operators ignore FMC speeds, and directly use those for the Assumed Temperature (and it's maximum TOW) straight from the Airport Analysis. My own company has a compromise, use the FMC speeds if within 3 knots of the Airport Analysis speeds, and use the latter if outside the 3 knot limit for any one of the 3 speeds.

One comment buried within the responses makes me acutely uncomfortable, that of using the Vr and V2 for the higher weight, but using the V1 for the lower actual weight. Good for the rejected Takeoff, but you're walking straight into the Devil's playground in the continued Takeoff case. There is a clear relationship within any set of V1, Vr, and V2's, the primary one being the capability to accelerate from V1 to V2 at 35 feet by the end of the TODA with one engine failed. The V1/Vr/V2 split in the AFM allows for this, but, if you're introducing a larger Delta V than certified between V1 and Vr, NOW you're inviting an extreme overrun. If you doubt this, take a look at the Wet Runway performance where the V1/Vr Delta is often much wider. Achievable?, YES, but only at a reduced weight.

FE Hoppy, If your aircraft wants to become airborne before V1, you're a pilot, you know how to stop it. It's not a problem.

Fly Safe,

Old Smokey

[alexban]

Ziggy:..'g' is the gravitational acceleration 9.8 mps.It's a fixed number.
Empty cruise: let's put it this way:
we have two TOW's -48 and 53T.
The available braking force at M=48T is lower than the available braking force at M=53T (F=M*g*mju g=9.8,mju =braking coeficient,the same in this case )
Now we accelerate both planes. At V1(48T) M48 will stop safely (available braking force=required braking force) ,M53 will also stop safely (available braking force>required braking force V1(48)<v1(53) )
We continue to accelerate:
At V1 53T (more than 10 kts higher in this case):
The M53 will stop safely -it's calculated that the available braking force=required braking force to stop the plane on the rwy
But in the M48 case, the available braking force it's lower than the M53 case (lower mass) .Indeed ,also the required braking force it's also lower due to the same reason.
But will the available braking force be higher then the required braking force? Why then the lower V1(48T)? My guess,maybe due to lower Vr and V2,optimum climb,etc...but it's a guess.
I think Old Smokey is right,but I would prefer to have some calculations done ,or the AFM saying this,not just some smart dispatcher(at our company).It says nothing regarding this subject on our AFM ,or FCTM.
Actually Boeing performance is giving us graphs where you use the assumed temperature you've found from the Airport Analysis ,the field elevation and ACTUAL TOW to find the take off speeds. But some smart guy is thinking now,that we should not use this tables,just the airport analysis,as I've told you.

One funny thing,which I've seen in real life:
Which will stop faster :a 70T tank or a nice Mercedes (with ABS,EPS...),both running at 80Kmph ?
You can bet on the tank,it's amazing.It is also on Discovery Channel,some documentary regarding new weapons.
Of course,different mass,also different mju,but nice to see ,though.
Brgds....
Alex

[FE Hoppy]

Old Smoke,
I'm a flight Engineer.
I did have the rather unpleasent experiance of sitting behind a pilot conducting a take off on a 3*-500 empty where he didn't prevent it until about 10' RA and then decided to put it back on the tarmac for a while!!!!!

When the RTOW at max assumed is almost twice the aircraft actual TOW I would be very wary of using speeds for temp.

[Old Smokey]

FE Hoppy,

Assumption is the mother of all screw ups, and I screwed up because I assumed wrongly. Sorry!

It was tongue in cheek, heavens above, I don't want to ever insult the greatest safety factor ever put in a cockpit - the Flight Engineer.

Regards,

Old Smokey

[FE Hoppy]

;)

I made the same mistake I assumed he would be aware of the potential problem:uhoh:

[mutt]

alexban

V1,VR,V2 should be obtained from the FMC or QRH based on the actual takeoff weight and the assumed temperature to be used for setting thrust. This is explained in the Boeing Performance Engineers Operations Course, the complete presentation is 5 meg and is available from http://myboeingfleet.com


Mutt.

[alexban]

Mutt: I've checked the site.The closest I find is PERFORMANCE ENGINEER TRAINING, a 266 MB file.I'll download this,but is this what you're talking about?
I couldn't find the Boeing Performance Engineers Operations Course,5 mb file.Where should I look?
Thks.
Brgds Alex

[BOAC]

Smokey One comment buried within the responses makes me acutely uncomfortable - I'm hoping you did not misread my post there!

Mutt - I think most airlines do it the alexban way for simplicity? Yes, it is the 'wrong' V1, Vr and V2 but as OS says - its a compromise.................

[Old Smokey]

No BOAC, it was nothing that you said, but actually one of alexban's where he quoted "The assumed 50C guarantees you'll stop for a TOW of 53T if you use the V1 for 53T.If you're lighter than 53T shouldn't you use corresponding v1?". My discomfort arose from the possible hint within the statement that the lower V1 might be used with the higher Vr/V2.

Quite possibly he was not inferring this, but I wanted to jump on it before someone thought it was a good idea. (See other forums, some have).

The background to some of the differences between Mutt's and my own company's approach is that, as Mutt has indicated on other occasions, his company does not take credit for Stopway and/or Clearway, and directly use the Balanced Field speeds for the actual weight directly from the FMC. We, however, do take credit for Stopway / Clearway, i.e. a non-balanced field which is not acommodated in the FMC, and consequently obtain this V-speed data from the Airport Analysis. For unbalanced field operations, the Airport Analysis is the only PRACTICAL source of data.

Regards,

Old Smokey

[Empty Cruise]

Hi Alex et al...

Had me physics book out in broad daylight for the first time in many years. No matter how I tried to set up the equation - it always came out with mass on one side and mass on the other side as well. I therefore - perhaps falsely :{ - see them cancelling eachother out exactly, i.e. when comparing braking force available with braking force required, the mass is not a factor, only mju & speed squared. Trying to do the calculation for the same speed (the V1 for 53 T) and mju, but varying the mass (48 T vs. 53 T), I always come up with brake force required being less than brake force available when keeping the speed for 53 T.

The lower V1 for 48 T - well, AFAIK, the FCOM will give you only data for V1min or V1improved, depending on what table you use, whereas the GWC will give you the relevant speed for the particular rwy (i.e. only use V1min when absolutely necessary due field length or close-in obstacles), and as OS said, take into account unbalanced T/O etc. In that way, we are perhaps comparing oranges and apples here? :O

And regarding the tank - yes, especially firing the gun with level barrel straight ahead & a set of new tracks will do the trick :D - you think I could have those as a re-fit on the Merc? :p

Brgds from
Empty

[mutt]

Now in Jakarta.... Alexban, go to Performance Engineers training, then course material, the operations course and reduced thrust. It should be there.


Mutt.

[alexban]

Thks Mutt,I found it.What I was looking for,indeed.Best time in Jakarta..
Brgds Alex