I have a question for the collective expertise here, that has come up on a small UK pilot forum I am a member of.

A member is doing his FAA checkride on a Seneca V he's recently started flying. He's noticed that the Accelerate Stop chart in the POH has the unusual property that ASD increases as weight decreases to a marked extent. In fact, extrapolating the straight lines would suggest a Seneca V overloaded by 2000lbs has an ASD of zero...!

This appears consistent over 20 years of Seneca V POHs from a sample of a few. See example attached below.


All the prior Senecas, including the very similar III and IV have ASD increasing with weight, as you'd expect.

He's written to Piper a couple of times and get the same reply


The consensus in our small group is that the PoH can't be right! Yes there is something of a lift effect that reduces braking (it's always the same 81KIAS reject speed at all weights, so significant lift on a lightly loaded Seneca V) but it can't be as extreme as in the POH and so contrary to the other earlier Senecas and all the other light twins we're familiar with.

Any views?

This effect has been demonstrated to be true in heavy commercial vehicles. More weight on wheels gives better traction and thus shorter stopping distances. Like you say though, there’s a point of diminishing returns, otherwise an extremely heavy vehicle would stop on a dime. My guess in your case is the straight lines are accurate enough for the relatively small weight range, but if they were drawn truly accurate it would be an arc that eventually went negative.

That is different than the Seneca III; in which the weight (middle) lines go down and to the right rather than up and to the right.

Does the V have Cleveland heavy duty wheels and brakes as standard?

Certainly illustrates that if you want to extrapolate outside of these charts (as some do when over gross) you could be making a huge mistake.

The lighter the weight - the less braking force you can get out without locking.

A variation of that problem can be seen in older Cherokees (with manual flaps) - raising flaps up shortens the roll out distance by putting more weight on the wheels and allowing stronger braking.

Raising flaps always puts more weight on the wheels and helps braking. But in a retractable the cost of reaching for the wrong switch are very high! Best limited to Cherokees and Cubs and such with fixed gear and “instant” manual flaps.

Some airliners have spoilers that kill the wing lift to make braking more effective. Failure to use these was a factor in the Little Rock airline crash some years ago.

I've always been bemused by the whole "raise flaps to get better braking" argument for light piston planes. Unless you fly something crazy like a carbon cub or highlander, you can easily get into places with no effort, that are then too short to depart from. If getting stopped is a problem for some pilots then the answer is to stop landing so fact - not to figure out ways to work your tiny brakes even harder.

Agreed. The speed on the chart is 81KIAS for the reject. Assuming the same AoA at all weights, the lift is constant wrt weight.

In the case where lift=weight, a very small brake application would be enough to lock the wheels.

But it still isn't clear to me why the Seneca V is so different from all other light twins I am aware of, including the Seneca III below (this example is a European model limited to 1999Kg).

Ever took part in landing short competition?
Ever had a cow wonder on the runway while you're already rolling out?
Or inattentive pilot taxing out right in front of you for intersection departure?

Has nothing to do with 'should I be landing there' and all to do with 'I've got this trick in my bag and I know how to use it'

Brake size? Wheel size? Tire pressure?
Any of that changed significantly from the other models?

If they upgraded brakes to handle higher gross weight it could have the effect of them being too strong when lightly loaded.

Yes, the same applies to electric flaps - they raise too slowly for it to have any effect.

The only planes I've done it in where it made sense to me were older Cherokees and Skyhawks with manual flaps.

yes to both. Quite frequently on the first. Flaps are the last thing on my mind. No practical benefit and lots of opportunity for things to go horribly wrong.

Brake size? Wheel size? Tire pressure?
Any of that changed significantly from the other models?

If they upgraded brakes to handle higher gross weight it could have the effect of them being too strong when lightly loaded.

I don't have the POH any more since selling my Seneca II a couple of years ago. Mine did have the heavy-duty brake option and I did flat spot the mains twice - both times during recurrent training demonstrating maximum performance stopping while light. My instructor wanted to see it. So we did it.

It did stop much better with the flaps up after touchdown. Johnson bar made that pretty easy.

The thing is if the brakes lock, the stopping distance goes way, way, way up. If the pilot has to modulate brakes, I can see that the stopping distance would increase.

But man, that Seneca V chart looks very suspect to me too!!!

My Columbia 400 also POH also says to put the flaps up on touchdown for maximum braking for shortfield landing. It also helps put more weight on the mains. Since there's no normal landing procedure or charts in the 400's POH, I make every landing shortfield procedure. It can eat up runway on landing if you let it.

Twice? What was learned the first time? Did the instructor pay.

Max performance stopping (to me) is abusive to the aircraft and needless risk and wear and tear.

My III had great breaks. I could easily lock them up with little effort. ( I think, never done it that much.) So I suppose the training reality is that if you want to learn to really do a max performance stop you will eat up some tires as you learn just how hard to press. If you don't lock them up, how do you know how much better you can stop.

Tires and brakes as training consumables. That's what rental aircraft are for!

Well, in the big scheme of things, a couple of tires aren't that big a deal. My CFI buddy is ex-military and we did a full performance stop every year. "Book says you can stop in this distance, I want to see you do it." All it takes is a little squeak and there's an instant flat spot. Not deep flat spots all the to the cords. My Seneca II also had VG's and was unusually light, in addition to the heavy-duty brakes.

Not so easy to rent a twin. And you have to train regularly to maintain proficiency.