Why this speed limit?

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minfremi@reddit (OP)

Haven’t checked the FAA charts, but it’s on the Jepp charts

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AdventurousBite913@reddit

"Boeing bounce" and ACN/PCN is my guess.

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I used to work in Boeing's Airport Compatibility office before flying for a living. Unfortunately I don't have access to a lot of the data I used to, but just by looking at it for a few minutes with what's publicly available there could be a couple of reasons. The spacing between parallel taxiways J and K meets Code F wingspan requirements so it's not really an issue. The clearance between the taxiway J centerline and the service road west of the terminal buildings is only Code E compliant, so it's possible that a taxi speed limitation was placed to reduce the chance and severity of lateral deviations. However that's probably not what drove this specific one because it would affect the A380 as well. The other likely candidate is pavement strength. Usually we limit taxi speeds to reduce pavement wear. A 748-8 while lighter, has higher ACNs than an A380. The extra gear bogeys do wonders im spreading the weight. Unfortunately US airports, and many worldwide airports in general, rarely publish taxiway PCNs so it would be something to ask the airport authority and I don't have that information to check umfortunately. It's unlikely to be related to jet blast, shoulder erosion, or FOD ingestion as we would usually word those notes in regards to idle or minimum power rather than taxi speed.

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craciant@reddit

Anecdotal observation here- A380s at KIAD always taxi with "wingwalker" service vehicles- 747s do not.

Neither type is often seen on taxiway J- both typically operate on the center runway. The cargo ramp is midfield, and of the operators that use these types for passenger service (Emirates, British, and Lufthansa) two of them use gates on the west end of terminal AB. (Emirates' gate is right in the middle)

30 is the prefered launch runway in general. The east runway (served by J) is prefered for corporate jets for both launch and recovery due to its proximity to the biggest FBO. The center runway is used to launch jumbos to minimize wake turbulence headway on 30 (its also 1000' longer)

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ItalianFlyer@reddit

That might explain why it only affects the 747-8. If the A380 is mitigated with wing walker vehicles, then there's no need for additional restrictions. As far as restrictions go a taxi speed limit is much less intrusive than requiring vehicles to follow you. When I used to do this, the argument I often made was that the 747-8 just barely intrudes into the Code F wingspan (or Group VI in FAA land) by a couple of meters per side. We could prove equivalent levels of safety without needing the full A380 mitigations. Some airports would buy that, and we could come up with less onerous restrictions, or some wouldn't and we would need the full monty.

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CBRChimpy@reddit

The distance of the outboard engines from the taxiway centreline. Wide enough and it will overhang things you don't want too much thrust near. So you limit speed to limit thrust.

The 747-8 has the outboard engines a little further away than other 747 models and every other passenger aircraft except the A380. The A380's are wider again, but the A380 is banned from that taxiway.

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Bunslow@reddit

So you limit speed to limit thrust.

That is not how planes or cars or trains work

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CBRChimpy@reddit

Maybe not in a vacuum with no friction.

But in reality, the higher the speed the more thrust required to maintain it.

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craciant@reddit

But in reality, airplane engines produce a lot of thrust at their lowest powe setting. We need brakes to slow down, not thrust to speed up.

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fighterace00@reddit

Wheels are pretty efficient, they've been introduced for a few years now

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CBRChimpy@reddit

I guess that’s why wheeled vehicles don’t burn any fuel if they maintain constant speed?

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fighterace00@reddit

Energy consumed is a function of time/distance. Force applied is a function of friction which is minimal.

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craciant@reddit

I always laugh when pilots try to talk physics. Alot of those words are indeed physics-y but the manner in which they are arranged is indeed gibberish :)

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Bunslow@reddit

my guy have you never driven a car before? higher speeds require high cruise powers, but in no car on the planet does cruising a highway require redlining the engine, nevermind slow speed urban taxiing.

at such slow speeds, the difference in power is entirely negligible. i mean that literally, teh design engineers of the pavement literally would be unable to discern the difference due to the extreme precision required.

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NevadaDoug1961@reddit

Then why not just NOTAM that 747 aircraft on taxiway J must keep outboard engines (1 & 4) at idle?

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randomroute350@reddit

This isn’t correct. It has to do with the load capacity of the actual concrete.

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Antrostomus@reddit

The 747-8 has the outboard engines a little further away than other 747 models

The publicly available Boeing Airport Planning documents show both the -8 and -400 outboard engines as 69ft (nice) from centerline, though.

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pontifican_t@reddit

Have taxied the 747 and triple, neither need any more than idle once you’re moving even at max gross. I would guess it’s more to do with braking and the damage that can do.

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Screaming_Emu@reddit

Maybe, but honestly even the heaviest 747-8 doesn’t need much extra thrust to taxi. Generally you just need a little bit to get you going and then there’s enough thrust at idle.

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ThatHellacopterGuy@reddit

A 747-8 close to MGTOW might be on the ragged edge of that taxiway’s weight-bearing capacity, and Airfield Management may have decided that a taxi speed restriction will help preserve the taxiway’s structural integrity.

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Many_Answer7575@reddit

Airfield pavement design is independent of taxi speed. The faster the aircraft taxis, the more lift it develops, reducing the effective load on the pavement. Speed limits on taxiway bridges are due to limited lateral clearance. Reduced taxi speeds reduce potential for aircraft wander from centerline. 30 year airport engineer and planner here.

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House_of_Rising_Moon@reddit

Super insight, thank you

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Ancient_Mai@reddit

I would argue a transport category aircraft taxiing develops zero lift. There is no lifting flow over the wing surface.

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Flapaflapa@reddit

Is the lift developed at 17 knots vs 25 knots at ~level (on tricycle gear) airplanes significant?

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Turkstache@reddit

Not on a 74 on the ground no way. That wing has a 2 degree angle of incidence, but flying level at even our highest speeds is another 1-2 degrees nose up. In fact even at our highest takeoff speed, there is no discernible sensation of lift or pitch up at T/O trim until you positively rotate. Otherwise, the jet is going to stick to the ground.

I plugged the data into a calculator. 17 to 25 kt difference is about 1000lbs in lift. With weights ranging from 400,000 to 1,000,000+lbs, that's a rounding error.

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Flapaflapa@reddit

That's kinda what I figured.

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Stunt_Merchant@reddit

LOL, nice :o)

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PontiusThe-AV8Tor@reddit

To an engineer asked to calculate something for the purpose of creating a limit and adding or subtracting safety margins. Then yes critical.

In real life where we may be plus or minus 10-20% of what we think we are as even with weight on wheels sensors in modern gen aircraft we still don’t really know accurately what we weigh by the time we’ve held and burnt fuel queuing and the factor in density and ISA variation and wind and something for grandma.

But the engineers, planners et al will have a reason for a figure like that it could even be above that speed the wing flexes X or Y amount when taxying on that specific type or because there are frangible objects in the path the outboard engines at the height of B748 wings versus A380 or other 4 engine types or the PCN doesn’t support A380 but can support B748 so that’s why it is type specific etc etc

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Stunt_Merchant@reddit

Lift is a function of the square of the speed.

So, all other factors being equal, 25^2/17^2 = 2.16.

So for an extra 8 knots you have twice the lift.

That seems surprisingly high and I would welcome a correction.

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mig82au@reddit

2 times fuck all is still fuck all. There's nothing surprising about your numbers.

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Flapaflapa@reddit

I don't think a correction is needed, that equation explains why approach speeds vary a little with weight.

Twice the lift of a pretty low number due to low AoA in taxi attitude is still a pretty low number. A 747 rotates at about 160 knots. I have no idea what change in coefficient of lift at taxi attitude to takeoff attitude is (one thread I saw quoted "0.3 to 1.25").

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Stunt_Merchant@reddit

Thank you :o)

0.3 to 1.25 seems about right. C_L will vary with angle of attack and wing geometry i.e. deployment of high lift devices. But in take-off it is angle of attack that changes as the aircraft rotates; the high lift devices having been configured before the take-off run begins. I heard it is more like 3.0 on landing with everything hanging out but not certain of that figure.

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Many_Answer7575@reddit

IDK. That’s a question for aircraft performance folks. We design the pavement based on fleet mix, forecast number operations, annual growth, and maximum taxi or takeoff weight of each aircraft.

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randomroute350@reddit

Has to do with harmonics and resonance as per a post below. Not lateral clearance and lift.

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Individual-Zombie-97@reddit

That part with lift must be a joke from your side, right? 😅

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PropOnTop@reddit

That took me by surprise too - he must have seen some 747-8's accidentally take off a taxiway at 21 MPH.

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SnazzyStooge@reddit

Maybe the same reason a lot of taxiway bridges have speed limits….

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Classic_Sock_383@reddit

This is the correct answer

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superspeck@reddit

… fools… so, Southwest?

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Classic_Sock_383@reddit

Don't forget Frontier. 😆

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michi098@reddit

Also if a faster plane slams on the brakes for whatever reason, there’s a lot of weight digging into the taxiway.

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shreddolls@reddit

Taxiway A in yyz gets substantial washboarding. Long down slope that ends with a 90⁰ turn. Unless you ride the brakes you pick up decent speed.

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randomroute350@reddit

This.

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Screaming_Emu@reddit

This makes the most sense to me

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TraxenT-TR@reddit

Good luck on operating out of IAD tomorrow. Don't get hit by those big weird busses on off road tires that drive around like maniacs there.

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swakid8@reddit

IAD isn’t that bad….. IAD based here…

IAD is actually the one of least drama filled airports and the moon buggy are a relic from the past.

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Horror-River-3861@reddit

I live 10 minutes from IAD and fly out of it nearly weekly. It's always a treat to get to ride in the moon buggies. Been riding in them for over 30 years now. Back in the day they used to act as a jet bridge and pull right up to the plane door, hence the ride height adjustment feature that they rarely use anymore

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Rainebowraine123@reddit

I flew in recently and those things really do seem to operate without rules! They were kinda intimidating to us in the CRJ. One nearly ran into us as we were pulling into the gate.

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SpiritFlight404@reddit

I would imagine it has to do force. Equal and opposite reaction. Either from the engines blasting around bits of rock or braking force to slow down further applying a force to the pavement materials. Either way they did math that limits damage.

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minfremi@reddit (OP)

The A380 used to fly into KIAD recently. What about them? Why is 747-4 not listed; shouldn’t it be similar to the -8?

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fighterpilot248@reddit

A380 still flies into IAD :)

Saw a Lufthansa A380 fly right over my head last week on short final.

(Also you'll see a regularly BA A380 come in as as well)

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Horror-River-3861@reddit

Plus Emirates has a daily A380 arrival and departure.

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uehara19sox@reddit

The A380 is not allowed on most of taxiway J. It’s only south of taxiway F. Likely the restrictions are unnecessary at that point given it’s not directly next to any terminals.

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OutOfBase@reddit

The A380 has a different landing gear footprint and therefore has different pavement interactions.

The 747-4 is lighter than the 747-8.

Someone ran the numbers and determined the 747-8 was the only airplane that needed to be restricted to avoid damage to the taxiway.

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dumbassretail@reddit

But a constant speed doesn’t apply force, an acceleration does.

You could get to 17 knots without ever applying high thrust. Similarly you could stop almost instantly from just a couple knots with bad results.

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talltaletom@reddit

cant believe theres another nerf to the 747-8

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nobody8008@reddit

It's due to destructive harmonics/resonance created by the 747-8 when it taxis above these speeds. It has to due with the mass of the airplane and the distance between the landing gear combined with the shape & size of the concrete squares that make up this taxiway. There are other airports with similar construction that share this restriction. Source: I'm a 747 instructor.

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pony172@reddit

Have been told something similar when I asked why there is a speed restriction in SDF for the -8 and not the 400. It was explained to me that at heavy weights the -8 can create a bow wave effect at higher speeds damaging the concrete over time. Most airports do not have a speed restriction for it. As a simple 74 driver I just follow the rules. They probably got written because someone dorked it up.

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randomroute350@reddit

This is the answer! Just had a someone explain this to me last week in SDF as well

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Dave_A480@reddit

Maybe that's the fastest such a large aircraft can go without risk of missing turns or not being able to slow down fast enough?

Kind of like how roadway curves you can easily take at 50 in a Miata are marked caution-25mph because a semi truck can't handle it at 50?

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cwleveck@reddit

So in case one gets rolling without a pilot in it they can catch it before it gets away. My wife did that with our Excursion once. Managed to start it. Forgot something inside. Left it running. In reverse. With the parking brake off. And locked herself out of it as she got out. I heard her screaming from the garage and came out just in time to see it easily making the turn as it left out drive way to go down a very long hill. Luckily the guy across the street has a nice retaining wall with his water main running just behind it to stop my truck before it ravaged someones house at the bottom of the hill. So unless they want to start building retaining walls all over airports..... And make it rain every time one escapes....

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JPAV8R@reddit

Taxiing the 747-8F at speeds much higher than that can cause sidewall damage to the tires. It’s recommended to allow the aircraft to break free with some thrust and then use almost idle thrust on the taxiway. Then allow it to accelerate and then slow it down with one continuous breaking application

Even at almost idle thrust you can easily exceed 20kts on even heavily laden 747-8F. Now since they weigh more than the -400 I imagine the stress on the pavement of getting to 20-25kts and then decelerating to 7-8kts.

Now before you speed police get upset it’s extremely easy to get the 74 to 20kts+ and it requires a good scan of your ground speed to know it. You are after all sitting three stories high in the air and that kinda warps your perspective of speed.

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CluelessPilot1971@reddit

It's obviously a school zone. They teach little Cessna preschool there.

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Calypso_maker@reddit

Probably a blind corner.

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turningfinal28@reddit

This taxiway likely does not meet full taxiway standards for 747-8 (ADG-VI) aircraft. In order to mitigate some of the risk of aircraft with larger wingspan and non standard object-free areas for that aircraft type, airport sponsors request a Modification to Standards from the FAA. Many times, a taxi speed limitation is sufficient to mitigate the risk such that taxilane standards can be used.

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