7
$\begingroup$

I am trying to figure out why is it that when you have an inoperative anti-skid system, the calculated V Speeds are lower?

I'm trying to see this from a performance perspective, could this be due to the brake energy limitation of the aircraft?

$\endgroup$
7
$\begingroup$

When you say it affects V speeds, I assume you mean V1 (and possibly VR), but not V2. V1 is usually calculated from the balanced field length, which is where accelerate-stop-distance and accelerate-go-distance are the equal:

Balanced Field Length
(image source: boldmethod > Engine Failure On Takeoff: Do You Stop Or Go?)

While the accelerate-go-distance will be unaffected, the accelerate-stop-distance is affected by anti-skid because you will not be able to brake as efficiently as with anti-skid operating normally:

the antiskid system stops the airplane for all runway conditions in a shorter distance than is possible with either antiskid off or brake pedal modulation.

[...]

Braking with Antiskid Inoperative

When the antiskid system is inoperative, the following techniques apply:

  • ensure that the nose wheels are on the ground and the speedbrakes are extended before applying the brakes
  • initiate wheel braking using very light pedal pressure and increase pressure as ground speed decreases
  • apply steady pressure and DO NOT PUMP the pedals.

(Boeing 737 NG FCTM 6.45 - Landing, emphasis mine)

As you can see from the above quote, you will increase the stopping distance compared to a normal braking with anti-skid, which results in a larger accelerate-stop-distance, which then reduces the balanced field length V1.

It has nothing to do with the brake energy limitation because the maximum energy that can be absorbed by the brakes is the kinetic energy of the aircraft at the moment you start braking. It does not really depend on how you brake, but with anti-skid inoperative there is a chance that the tires will lock up while decelerating which actually reduces the amount of energy going into the brakes and increases the energy that goes into the tires (in the worst case, this energy will then cause a tire blowout).

$\endgroup$
1
  • $\begingroup$ It's generally a given that tires will be blown if any sort of hard high speed braking is attempted with AS off. It's impossible to modulate the pressure with any accuracy to get close to lockup without going over. $\endgroup$
    – John K
    Oct 13 '20 at 16:26
0
$\begingroup$

with inop anti- skid, application of full brake press may result in wheel lock, skid and aquaplaning in certain situations. ineffective braking would entail larger distances : TORR (Take off) or ASDR (reject take off). for the same field length, temp, elev and AUW you have to reduce v1/vr ratio which would in turn reduces V1. this is for safely stopping the aircraft in case of reject take off within the ASDA.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.