Correct. The 727-100 was the only type I ever heard of with nosewheel brakes. And as I said, those quickly became vestigial.
As you may know, there are efforts now to integrate an electric motor into nose or main wheels to enable taxi without the main engine(s) running. If the engineers ever get it right, there will be significant cost savings not to mention some legit greenery and an opportunity for even more green-washing.
If indeed nosewheel motors become common I’d not be surprised to find them doing a regen braking step during retraction. Not for power extraction, but just as a way to stop wheel rotation without needing to scrape expensive tread off the tire(s) every time.
It would have to be an impressively quick turnaround for the brakes to still be really hot from the landing.
Question for our professional pilots: Is there a max brake temp limit for takeoff? (I’d imagine so, in view of the possible need for heavy braking after a rejected takeoff.)
There is. There are also limitations on quick turns after a normal landing. The details vary from plane to plane of course.
The limitations are really on brake energy, which manifests as temperature. The brakes can only absorb so much energy converted to heat over a time interval without getting too hot. And once too hot both their stopping power degrades, and there’s potential for fires & burst tires.
Some airplanes have brake temperature gauges, others have overtemp warning lights or messages, and yet others just rely on a nomogram or table based on how much time has elapsed since the last major brake application (post-landing or rejected takeoff) vs weight at the time and air temperatures. Look up today’s facts in the chart and determine whether it’s “go”, “wait awhile”, “call maintenance”, or “call the fire department!”.
A long taxi-out coupled with poor brake technique can get both brakes and tires pretty darn hot before takeoff even if they were cool before pushback. That situation can be detected by gauges & idiot lights, but are ignored (or built in as a fudge factor) on the nomograms.
For scale: A heavyweight high speed rejected takeoff can lead to brake cooling times of an hour or two before taxi would be safe. Conversely, following a normal landing and ground turn around time of an hour-ish, the brakes might be down to ambient or just warm to the touch. So effectively “empty” of stored energy and ready to absorb the full rated amount.
On a typical mid-day preflight of a new airplane you can sure tell which prior pilots ham-fisted (ham-footed?) the brakes and who did not. Sometimes they’re still scorching hot when you get there, and other times they’re barely warm. Holding your carefully calibrated hand nearby to feel how much radiant heat is flowing off them is a regular part of the process.
In my latest bizjet there was no brake temp sensor, so we had a limitation in the event of a rejected takeoff. Depending on the speed at the moment we aborted, we had to either wait 25 or 45 minutes before attempting another takeoff. Or above a certain speed, we had to have a brake inspection by a mechanic before another takeoff.
The trouble with this was it relied on one of the pilots registering and remembering the speed at a very busy moment. In a rejected takeoff they’re taking several quick actions, running memory items and then checklists, and talking to ATC. Very easy to not notice the precise speed at which it happened.
No, the Trident is the one I was thinking of. I wonder if it needed a separate line on the ramp when it was being parked at a terminal gate. I think main gear was also unusual; four wheels abreast on each side, and rotated 90 degrees during retraction. Complicated, but may have been more compact and less scuffing during turns.
Update: I was all ready to go to check it out but never saw anything more about it. Turns out they blew an engine on the first leg of the trip. According to the Times Colonist story, the plan is to swap out an engine from the Hawaiian Mars and make another attempt later this month.
Question I’ve been meaning to ask the commercial pilots. Can they just spray water on wheel assemblies when they’re hot or is that going to damage the disk plates? I ask this because the brakes have to deal with rain.
The pilot taxiing (captain at most carriers) aligns the center of the airplane as they perceive it with the stripe. The can’t see, and don’t care, where the nosegear is. Once the marshaller starts waving their wands the pilot takes that guidance unless it’s wacky.
Meanwhile the marshaller could just be steering a conventional centered nosegear onto the stripe. Or they could be steering what they perceive as the centerline of the airplane onto the stripe, with the nosewheels’ exact position being fine-tuning of that bigger picture. If the latter, an extra stripe for an off-center nosegear would be maybe nice to have, but probably not need to have.
I’ll also point out that the goal isn’t to get & keep just the nosegear on the stripe. It’s to get the whole airplane parallel to the stripe and the nosegear on it. Nosewheel on stripe but airplane cocked in the space is a fail. That suggests to me the marshaller is watching the bigger picture and nose and tail ats much or more than the exact nosewheel position. At least in the early phase of docking; by 5-10 feet remaining to the stop point the die is already cast as to the bulk of the fuselage alignment.
The airplane was designed for dirt runways in Africa & the Mideast. The multiple tires were about increasing the contact patch and reducing the ground pressure. As to why side by side versus fore-aft I have no clue. Lots of experimentation back in the day. But as a guess … if you assume a softish surface, having a fore-aft arrangement means that if the front pair of tires are leaving any sort of a rut, the rears will drop into that rut and make it even worse.
No clue what the fire dept procedures are. Big difference between a little rain and a firehose blast. @KCB615 was a firefighter and I believe worked at airports at least some of his career. Perhaps he can enlighten us all.
Direct hose streams on hot brakes are an absolute last option. Usually hose streams are only used if there is actual burning taking place on the wheel assembly. Direct water application will rapidly cool the brake assembly; depending on the metals involvedand how hot they were when water hit it. At least some hardness changing will take place, if not outright shattering of components. If it’s so hot that stuff is on fire, the crystalization thing is moot so it’s “agent away” if there’s a fire.
We also don’t like to put personnel next to hot wheel assemblies. There’s a fusible plug that’s supposed to melt when the associated rim gets too hot that will let the pressure out of the tire. That plug can launch with some speed behind it, and can do damage to the unfortunate new guy on the nozzle who is up close and personal with the wheel. Tires can also burst, also with a bang. The danger area for the fuse plug is inside 45 degrees left and right. The danger area for the tires is inside 45 degrees front and back. Doesn’t leave much room for a sweet spot to stand.
There are some YouTube videos of wheel fires after RTO testing. They show the excitement of a wheel fire pretty well.
The better option is to use a fan to cool the brakes down if needed (as long as there isn’t a fire). Airbus products have a fan in each hub that will come on automatically if needed (of course it’s automatic on an Airbus…), and will turn off when the temperature comes down. We used to see/hear them running quite often. Boeings do not have the fans, neither do the smaller aircraft like Embraers or Bombardiers. Most airport fire departments that receive larger aircraft (Index C or larger) carry fans for smoke removal. We simply set the fan up about 8 or 10 feet from the wheel and sit back for 20 or 30 minutes.
We would measure brake temperatures if asked using a thermal imaging camera. The temperatures would continue to rise for about 10 to 15 minutes after the aircraft landed (or RTO’d, we had several of those on my watch). They’d frequently get scary hot from our perspective, but we’d be in communication with the flight deck who usually wasn’t too worried. We had actual numbers for temperature, they always said all they had was “ok” or “too hot” coming from their instruments.
Our favorite method is just to wait out the temperature rise. It always comes down naturally at some point.
The only way to remove heat with water is volume. Each gallon of water removes about 9400 BTUs if 100% efficient, which it never is. Flowing 95 gpm from an ARFF vehicle handline will cool down a brake assembly far quicker than 1 gpm from a pressure washer (or it’s equivalent).
The aircraft manufacturers, for all of their skimping out on stuff, do a remarkable job making landing gear that can take some serious thermal abuse. In two decades of playing with airplanes, I’d say there were probably 40 or 50 wheel/brake incidents at my little airport and none of them ever turned into an actual fire. Tires popped, rotors warped (looking at you, MD-80s), but never had a fire.
I remember when I was a kid and I noticed the yellow line painted on the ground at the gate, with markings for where to stop the nose gear for each airplane type that could be parked there. I wouldn’t say the planes I saw parked were inch-perfect, but not too far off. Impressive, for something the size of an airliner. At the time, I figured the jetway must not move very far if the plane had to be parked so perfectly. Maybe that’s not the limiting factor, though. There must be fuel hoses and other things that have to connect to the airplane when it’s parked.
I hadn’t heard that about the Trident, but it makes sense given the time when it was designed. I know the 737 had some optional features that allow it to use airports with limited infrastructure, and even gravel runways. The problems with the new -MAX variants trace back to some of those early design priorities.
As understand it the idea behind misters is to break water down into smaller particles so they can displace oxygen and convert heat faster. I would think this ideal to avoid damaging brake rotors. Unless it’s on fire the object would be to lower the temperature.