1
$\begingroup$

Let's say the current outside air temp is 15 C and you have decided to use 50 C as the today's assumed temperature for your takeoff. In this case, use of the T/O weight the assumed temp thrust supports gives you the balanced V1.

But our company rules are to use the T/O speeds of the "acutual" T/O weight instead of those that the T/O weight the assumed temp thrust applied provides. I assume this rule has been made to give a greater safety margin for a runway overrun in the event of a possible rejected takeoff at the expense of decreased obstacle clearance, since use of the actual takeoff weight invariably brings lower V1, Vr, and V2 speed than when the T/O weight of the assumed temp thrust is used. My question is: is it a common practice among commericial airlines to use the actual T/O weight instead of the assumed temp T/O weight?

Improve this question
$\endgroup$
10
  • 1
    $\begingroup$ Assumed temperature is simply a way to reduce your thrust to some predictable amount that you can still find in the charts. I've never heard of using an assumed weight for 737 performance. Fictitiously adding weight reduces assumed climb performance, which serves no good purpose. Can you give an example of how much you see "assumed weight" differ from actual weight? $\endgroup$
    – Ralph J ♦
    Mar 1, 2018 at 15:02
  • $\begingroup$ Finding the weight (at whatever thrust setting you're using) that produces a balanced field isn't about "assuming" the airplane weighs that much, it's simply determining how heavy you could be to still meet all performance requirements. Base your takeoff & climb speeds on what you DO weigh + whatever thrust you ARE using (even if that thrust setting is being reduced based on an assumed temperature). No good comes from using a low V1 and high VR with a light jet & long runway just because that's the V1 & VR that you'd have with 40,000# more weight! $\endgroup$
    – Ralph J ♦
    Mar 1, 2018 at 15:11
  • 2
    $\begingroup$ In my understanding the idea of the assumed temperature method is to calculate the whole takeoff as if it was as hot as you‘re assuming. That gives some margin on the day due to the true airspeed (and thus distance covered in a given time) being a bit lower than assumed. Mixing normal temperature speeds into it, especially V1, might not be conservative (I‘d have to think about it a bit more!). The only exception are VMCG and VMCA which have to be calculated at actual temperature, obviously. By the way, whether it’s a common practice or not won’t be easy to answer - only whether it’s safe. $\endgroup$
    – Cpt Reynolds
    Mar 1, 2018 at 21:01
  • $\begingroup$ @Cpt Reynolds "Mixing normal temperature speeds into it, especially V1, might not be conservative." At the balanced V1, the accelerate-stop distance and accelerate-go distance equal to each other. As the V1 decreases from the balanced V1, you need a less accelerate-stop distance, while you need a greater accelerate-go distance. Granted that's the case, doesn't a lower V1 always give a greater safety margin at least in terms of an overrun in the event of a reject takeoff? $\endgroup$
    – lemonincider
    Mar 3, 2018 at 8:34
  • 2
    $\begingroup$ @lemonincider Yes indeed. I would in that case be concerned about the accelerate-go case, where the aircraft has to complete the engine-out takeoff from lower speed than the balanced full-thrust V1, but only using reduced thrust. As I said, it’s entirely possible that’s OK, but it doesn’t feel right to me. I would have to think about it some more before being certain. $\endgroup$
    – Cpt Reynolds
    Mar 3, 2018 at 11:47

1 Answer 1

Reset to default
-1
$\begingroup$

You determine the t/o thrust as a function of t/o weight and runway length Once the t/o thrust is determined you can select the assumed temperature on the thrust limit page on the CDU.

V1 is not the target primary parameter, it is a consequence of the calculation, on modern FMS your calculated value is cross checked with a range calculated by the FMS, that is your calculation could be rejected by the installed FMS unless you may force your value on some models.

A minimum rate of climb is necessary with a speed of V2+10, and at worst V2 with engine failure.

Backward of this Vr is computed.

Improve this answer
$\endgroup$
4
  • $\begingroup$ I don’t think that is an answer to the question...? $\endgroup$
    – Cpt Reynolds
    Jun 11, 2019 at 17:42
  • $\begingroup$ @Cpt Reynolds, As far as I know no pilot will decide to use the assumed temperature ahead of every thing. This is a decision that is taken once you have already calculated the required thrust. On the other hand V1 is a consequence of V2 which itself is a consequence of the weight. So after calculating the required minimum thrust, then you may decide to use an assumed temperature if possible. So I am answering to the question, otherwise why not? $\endgroup$
    – user40476
    Jun 13, 2019 at 8:07
  • $\begingroup$ Well, the decision to use assumed temperature or not could influence the value of V1, maybe even V2 in some cases. The question was whether it is common practice to use non-assumed-temperature speeds when flying an assumed temperature takeoff (this question only makes sense for cases where the speeds are different between both options). So I don’t think explaining the calculation process answers the question about how commonly a different „mixed“ process is used... $\endgroup$
    – Cpt Reynolds
    Jun 13, 2019 at 9:06
  • $\begingroup$ Of course you have to understand the calculation process otherwise you will not understand why high assumed temperature will be rejected as not producing a compatible V1 $\endgroup$
    – user40476
    Jun 13, 2019 at 9:29

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .