Braking vs. Reverse Thrust

[Just another canuck]

Which is harder on the machine? I understand you want to avoid reverse in a low-wing or single like a Caravan on gravel, but what about pavement for these machines?

I've worked at a company where reverse is prohibited unless an emergency called for it and I've worked at companies where reverse thrust is encouraged and using the brakes is discouraged entirely.

Let's here what avcanada has to say on this topic...

... also, on a bit of a side note, does anyone know if heavy braking on the Caravan actually increases brake life? Is this just for breaking them in? Or is it true for the entire duration of the brakes life? Is heavy braking for a short time better or worse than light braking for an extended time?

[V1 Rotate]

I use both judiciously on both the Caravan and Metro and before that on the Cheyenne. Neither company had any rules about using or not using either. All pavement ops though.

In my opinion the reverse feature was designed into the system and can be used within normal parameters. Abstaining from reverse would be like not using the reverse gear in your car for fear of wearing out the transmission. Brakes will wear out... Kind of like gas will be used up when you fly...

Did fly with one guy who kept on using reverse kind of like differential braking, didn't quite get that especially since the Metros have a hydraulic nose wheel steering system that can to over 60 degrees either side of centreline


OOPS wrote all that, and just realized it's asking about parking and taxiing and not landing. Well it all still applies.

[HuD 91gt]

Which is harder on the machine? I understand you want to avoid reverse in a low-wing or single like a Caravan on gravel, but what about pavement for these machines?

Why can't you use reverse on a Caravan in gravel(of course not when down to taxi speeds)?


I myself barely use either one. Beta to get slowed down, brakes to bring you to a stop. Unless you are doing some serious off strip, or just wanting to stop by Alpha :p Of course once you get into some heavier machinery it is required, but something like a Caravan, I never did.

[Doc]

Caravan brakes are relatively cheap. Use the brakes on the 'van. Props are really expensive. Beware the gravel.
Compound brakes are much more expensive. Use the props more than the brakes when on pavement.
There is no correct answer here. Depends on the aircraft, and the surface material.
Landing on gravel, I'll bring the props back to a little more (less?) than flat pitch to initiate a "slow down" then, it's very light use of brakes. I don't mind using most of the runway. There's no prize, other than very expensive repairs to "making" the intersection. Nothing to "prove" here, either.
Taxiing on gravel....brakes. Trick here (you all know this) is to maintain forward momentum. Don't even taxi till you are sure you will not need to stop before your takeoff roll.
Circumstances alter cases.

[HuD 91gt]

.

... also, on a bit of a side note, does anyone know if heavy braking on the Caravan actually increases brake life? Is this just for breaking them in? Or is it true for the entire duration of the brakes life? Is heavy braking for a short time better or worse than light braking for an extended time?

If i remember correctly, somewhere in the manual it stated with the type of pads in the Caravan, heavy braking is preferred over light braking. I honestly don't remember why, but it's in their somewhere.

[Just another canuck]

Why can't you use reverse on a Caravan in gravel(of course not when down to taxi speeds)?

I agree... I guess I should have been more specific.

Thanks for the comments, Doc... what is your opinion on parking? ... using use of reverse to come to a complete stop that is... obviously on pavement only. I was talking to a co-worker the other day who says he pretends like the brakes do not exist...

If i remember correctly, somewhere in the manual it stated with the type of pads in the Caravan, heavy braking is preferred over light braking. I honestly don't remember why, but it's in their somewhere.

I thought so too... I remember reading it was well and the same co-worker as above didn't believe me. Can anyone explain this/elaborate?

['effin hippie]

Caravans go through pads pretty fast in my experience, manual be damned.

Saw 3 failures in one summer due to the pads wearing so much that the pucks traveled all the way clear of their recesses and spewed out the fluid. This was at an excellent operator too. Maint reached the point where they just changed the pads every 50 hr, since it was a crapshoot if they'd go through 2 insp cycles.

Twice I just lost the brake on the one side, once the puck jammed on an angle locking up the brake. Dishing on the disc makes it much worse too. It's worth a close look on the walk around.

More generally, brake pads are a lot cheaper than props, but when there is any loose snow, I try not to use brakes at all during the taxi, cold brakes = no melted snow = no freezing.

Plus everything Doc said. Vans have an almost supernatural ability to suck up gravel.

ef

/edit/ Maybe heavy brake application forces the disc to wear more evenly.

[Benwa]

When taxiing, I use beta/reverse up to the point where RPM starts going up again. Not more than that.

That's on a KingAir B100 on asphalt. On gravel I'll use a bit of reverse just after touchdown, if I need to make it as short as possible... never ever above 70 kts.

Ben

[HuD 91gt]

I thought so too... I remember reading it was well and the same co-worker as above didn't believe me. Can anyone explain this/elaborate?

Just open up the manual and show him. It's blatantly said right there. It may be a note, so read it all. Something about light braking being detrimental in pad life. I only have access to a "Turbine Training Center" manual, which is fairly light on information.


As for 'Van's blowing through pads. We had three Van's and as far as I know, none ever had any frequent problems?

[Doc]

On dry pavement. I use the brakes/props combination. Brake at the end. Shut down with brakes on, but I don't set the park brake.

[SuperDave]

When flown and taxied properly a Caravan should not use much reverse or brakes under normal circumstances.

After touchdown reverse is useful sometimes to take the top-end off a bit, (remember, aerodynamic braking is most effective at high speeds) but BETA will usually suffice unless you're going into short ice strips etc.
Taxiing properly on gravel seems to come slower to some folks than flying. Unless you're taxiing around Pearson, you're not going to save much time if you go 15 knots instead of 10. So, keep the taxi speed under control and plan far ahead. Use BETA as required, but NO reverse. If you need to step on the brake a bit to make a turn, aint no thang. Really. You can slow to a stop with a bit of BETA and a bit of brake or; once you get real comfortable and it's not a tight apron you can shutdown, feather and just coast to stop on the pad a-la-floatplane style.

Don't forget to sweep.

And, as is common knowledge, and re-iterated by Doc...make all your calls before you start moving...which in the Van can be easily done by smooth throttle operation, or by moving the condition lever towards high-idle. Rocking the elevator sometimes helps gets you going and reduces the amount of power you will need to start moving forward.

[HS-74]

Discing on the 748 is highly effective, brakes are used for taxiing and under 40 knots only. thats sop in our company

[ScudRunner]

Just . er into full reverse in the Flair .

[flying4dollars]

Depends on the runway length too. Coming into a 10,000 foot runway for example, in a Beech 1900 I'll only ever throw it in ground fine to slow down, below 60kts I'm at idle and the plane does a good enough job slowing down on its own with minimal to no braking needed as I turn off onto the taxiway. I'll use ground fine to slow down when taxiing at a decent speed rather than constant braking, and I use brakes to come to a stop (obviously).

On gravel, again, even using ground fine is more than sufficient to slow me down without having to lead foot the brakes (in a D model). In a C model, on touchdown, I'll pull the levers all the way down to reverse and as soon as it touches the bottom of the quadrant I pull it right back out and that's usually enough to slow me down quite well without the blades fully engaging in reverse and doing any prop damage.

[dashx]

From a replacement point of view I would rather inspect brakes on regular basis then change them rather than changing an engine.

And I have had the pleasure of changing both. I prefer changing brakes.

It took the "cowboy" a few years of constantly using thrust reverse on a PT6 Dash X before the gearbox finally seized on him one day. The chief pilot blamed the engine saying there was a known fault with the design (ok ya right....)

The next chief pilot put a memo out telling pilots not to use brakes and instead use thrust reverse to "save" brakes. (ok ya right). I think the DOM put a stop to that.

I think its the old adage: You can pay me now or pay me later.

I can stock quite a few brakes before it adds up to one engine.

[wabano]

For the purposes of take off and landing calculations, reverse is NOT taken
into account...IT DOES NOT EXIST!!!

Woe the pilot who delay a flight to get his reversers fixed!

For the big boys flying the old stuff in hot and high conditions,
reversers are your last resources in extremely thin margins.
Seen a heavy take off or land in say, Bogota?(8300 feet)
Need all the runway, in principle calculated WITHOUT the reverses!

So, always use the reverses to the max, but keep in mind
that they're your last margin in tight spots.

[Flying Nutcracker]

Having flown an airplane with a PT-6 or two on it I once asked the question regarding use of reverse to a maintenance engineer with a long time experience on the engine. His comments were that it is not worth using reverse for taxiing or parking when you consider the overhaul cost of said engine because of turbine blade damage from blowing up all the small dust particles (we're talking paved surface here!) in front of the engine intake just to get sucked right through the turbine.

Brakes and prop blades are not a consideration costwise in his professional opinion compared to the cost of a turbine engine!

Just thought I'd throw that out there...

[Donald]

I'd like to add, for those of you new to the PT6's who like to use reverse to back the airplane into a parking spot: have a look at your temp's when you do this. Watch them spike, and then remember to cool the engine at idle before you shut it down.

As far as reverse/braking during taxi and parking, common sense should be enough.

[pelmet]

[quote="wabano"]For the purposes of take off and landing calculations, reverse is NOT taken
into account...IT DOES NOT EXIST!!!


[img]http://farm4.static.flickr.com/3622/363 ... 6a5103.jpg[/img][/quote]

Not always true actually.

[Liquid Charlie]

In europe most airports have restrictions on use of reverse - some completely and others idle reverse only -- since no performance is based on the use of reverse (except for maybe an MEL item) it's always a bonus.

I have watched guys overtemp a pt6 using reverse rather than breaks to control taxi speed - yxl is a great example where this can happen

[xsbank]

Small point on brakes - steel brakes, use 'em as little as possible. Wear is directly proportional to use. Carbon brakes are completely counter-intuitive: cold braking/riding the brakes on the taxi out causes 80% of the total wear. A regular 'service stop' that is using them to slow down on a normal landing and taxiing in with them hot causes 20% of the total wear. Cold=bad, hot=good.

That is all.

[dashx]

In europe most airports have restrictions on use of reverse

Australia too.

I remember the first time they allowed the pilots to use reverse with a Dash at a gate. The airport manager had to come and watch because they were afraid we would blow the LD containers into the terminal bldg.

Eventually they relocated us to another part of the airport. no reverse no pushback...

Just a lot of walking.....

LOL...

[TwinOtterLover]

So far it seems like brakes are winning out in this and i'm not really gonna go either way all i know is that our pilots use reverse depending on the situation. such as if your landing on a gravel strip with a caravan, try to use the brakes so you don't eat the prop apart while if your landing on pavement with a twin otter go ahead with all the reverse your want just watch the torques. so far we have not had much of a problem with this except for one pilot in a turbine otter that used reverse so much there was soot on the blades.

Edited once: haha sry i had my moment for the day there and realized i wasnt really focusing on taxiing and parking. we use reverse more where i am for taxiing and a combination of both to park.

[PanEuropean]

On the aircraft I am most familiar with (the Twin Otter), reverse does not have much effect below about 60 knots... brakes are more effective than reverse at speeds below 60 KIAS.

Generally speaking, I think that brake pads are cheaper to replace than prop blades. By this I mean when reverse is used at low speeds, there is a risk of the propeller getting dinged from small bits of debris that are thrown forward.

Using zero thrust (in other words, moving the power levers aft of the idle stop, but not sufficiently far aft to cause Ng to pick up) makes sense during prolonged taxi, to avoid having to 'ride the brakes' to control taxi speed. But, that's zero thrust, not reverse.

All of the above presumes a dry runway with good braking performance.

I think that the risk of FOD to the prop arising from use of reverse would be much greater on singles such as the Caravan or PC-12, simply because the propeller blade is so much closer to the ground.

Michael

[rsandor]

I'm a bit of a brake guru when it comes to racecars, and much of it carries over to airplanes.

For those that don't know: when you first install new pads or shoes on a braking system, you are supposed to "bed" them. Which means you force the new pad to transfer (deposit) some of it's material onto the rotor or drum. Brake pads do not work well upon a clean iron rotor. They are designed to grab against a layer of their own material which has been bedded or deposited on that rotor or drum.

As was stated above, traditional pads work very well when cool, and as you work them, they heat up. Once they get excessively hot, they become ineffective. (you get a layer of gas between the pad and rotor, which makes the pad 'glide' over the rotor and doesn't get traction - and you can melt the layer of bedded material into a 'glaze' which is too slick to grab onto the pad) ...and if you have hyrdaulic brakes, you can boil the fluid too.

Carbon-ceramic braking systems are a little different in that they are much lighter physically (and also much MUCH more expensive) and they actually work better when they are hot, although maybe not quite as well when cold.

However, even if you have carbon cermaic brakes; at very high taxi speeds you have to be super careful with brake application because it's easy to over power them and lock them, especially if you are not 100% weight-on-wheels.

That said, it's also not good to apply too little pressure to the brakes at higher speeds. (this is generally what causes 'glazing') You want to apply 80-90% of the amount of maximum braking pressure possible. 100% being enough to actually LOCK UP the tires. On a race car I'd be suggesting 98-99% but on a plane that's just too unsafe to even try because it's too easy to lock-up and flat spot and or blow a tire. Anything less than 80% means you are using too much runway and not enough brakes, and anything less than 50% you are just glazing your pads and not slowing down at all. In my experience with airplanes, it's really hard to know exactly how much % of braking ability you are using, until you've actually locked up a tire or engaged ABS, which is NOT something you want to ever do in an airplane, if you can help it.

For pete's sake please do NOT stand on the brakes after a long hard brake unless you absolutely HAVE to, to keep the plane from rolling forward into a ditch or building or another runway. If the rotors and pads are hot enough, the pad will permanently leave some of it's material deposited on that section of the rotor. This which will leave a high spot on the rotor, which manifests itself as severe vibration when you apply the brakes. It might neccessitate machining or replacement of the rotor.

Regardless of all that, follow your SOPs. Some SOPs might sacrifice brake longevity in the name of safety and performance, which makes perfect sense. I'd be more wary of SOPs that try to make you save brakes (which are relatively cheap to replace) in stead of leaving you more runway for roll out.