HUD Airspeed

[Alphamale]

Why is the HUD airspeed indicator always showing 48 kts while jet is stopped on the ground?

[Tholozor]

Just the way the pitot is calibrated, it's the same way on the A-10C. The system uses the air pressure going against the plane to determine the air speed, so if you're not moving fast enough, it can't generate enough pressure to give the system a valid reading. 48 knots is probably just the slowest speed it can detect.

 

Edited June 13, 2018 by Tholozor

[fitness88]

I got called on my taxi speed being too high, how can we know we're doing 45 before getting booted out?

[IvanK]

Look at your Ground speed on the HI

[ouPhrontis]

I got called on my taxi speed being too high, how can we know we're doing 45 before getting booted out?

 

Lookout, look out towards the wing tip and try to picture someone attempting to run alongside, it's too fast if you'd not be able to keep up. It's poor practice to use ASIs or groundspeed readouts for taxi speed, especially so for pitot ASIs as they're unreliable at slow speed/on the ground.

Edited June 14, 2018 by ouPhrontis

[Nosferatuwhisky 1-1]

Also, don’t taxi at more than 70% power.

[26-J39]

I got called on my taxi speed being too high, how can we know we're doing 45 before getting booted out?

 

 

48kts is 55mph... try looking outside the pit for a speed reference :)

[fitness88]

Look at your Ground speed on the HI

 

 

 

HI?

[Yoda967]

I think he means HSI.

[fitness88]

I think he means HSI.

 

 

 

Thanks, yes I see it there.

[Midnightzulu]

Lookout, look out towards the wing tip and try to picture someone attempting to run alongside, it's too fast if you'd not be able to keep up. It's poor practice to use ASIs or groundspeed readouts for taxi speed, especially so for pitot ASIs as they're unreliable at slow speed/on the ground.

 

Ground speed is used all the time in the real world as a taxi speed reference. Also the whole “fast walk” thing is great in a Cessna at the local patch but will get you yelled at at JFK or LAX. 30kts taxi speed is pretty common on a parallel taxiway. Unless you’re Southwest. Then it’s about 50. :smilewink:

[ARM505]

The Boeing 737 FCOM's, FCTM and limitations section give no limitation on taxi speed, only recommending that 'if taxi speed exceeds 30Kts, use caution when using the tiller'. Or something like that (working from memory here)

 

That's my story, and I'm sticking to it :D And yes, GS readout is used all the time I reckon. Gotta know just how fast is too fast....

[ouPhrontis]

Ground speed is used all the time in the real world as a taxi speed reference. Also the whole “fast walk” thing is great in a Cessna at the local patch but will get you yelled at at JFK or LAX. 30kts taxi speed is pretty common on a parallel taxiway. Unless you’re Southwest. Then it’s about 50. :smilewink:

Well I have no idea about the airliners, or other large aircraft, just referring to the small stuff (Perhaps some of the large flying tubes have mechanical speedo drives in the main wheels or some other reliable metric, too?).

 

Ground can moan all they want; pilot is in command (so long as no-one's stopping for a picnic), though having said that I do recall taxiing at something approaching 30kts on the straights.

 

Not disagreeing by-the-by, I flew at Hurn airport which was quite busy for learning and they always tried to get us out the way in a prompt manner.

Edited June 14, 2018 by ouPhrontis

[Midnightzulu]

Well I have no idea about the airliners, or other large aircraft, just referring to the small stuff (Perhaps some of the large flying tubes have mechanical speedo drives in the main wheels or some other reliable metric, too?).

 

Ground can moan all they want; pilot is in command (so long as no-one's stopping for a picnic), though having said that I do recall taxiing at something approaching 30kts on the straights.

 

Not disagreeing by-the-by, I flew at Hurn airport which was quite busy for learning and they always tried to get us out the way in a prompt manner.

 

GPS/IRU is more reliable than the speedo in your car.

 

Here's a true ATC story: I was a brand new, very green LT just out of pilot training. My first assignment was flying DV (VIP) Airlift out of a fairly large international airport in Europe. On my first flight at my new base I was taxiing out to the runway on a parallel taxiway. I had been taught to keep it under about 15kts at the schoolhouse, so that's what I was doing. Apparently we caused quite the traffic jam behind us with a Lufthansa, a Delta, a Thomas Cook, and several others behind us. At some point Lufthansa decided they'd had enough and queried Tower as to whether the Lear in front of them could pick it up a bit. The IP in the left seat had a good chuckle and told me I taxied like his grandmother. Lucky for me, the Euros are pretty civilized when it comes to ATC. If that had happened at JFK, I have a feeling they'd have found a nice quiet place on the airport for us to wait as the conga line went by!

[ouPhrontis]

GPS/IRU is more reliable than the speedo in your car.

 

Here's a true ATC story: I was a brand new, very green LT just out of pilot training. My first assignment was flying DV (VIP) Airlift out of a fairly large international airport in Europe. On my first flight at my new base I was taxiing out to the runway on a parallel taxiway. I had been taught to keep it under about 15kts at the schoolhouse, so that's what I was doing. Apparently we caused quite the traffic jam behind us with a Lufthansa, a Delta, a Thomas Cook, and several others behind us. At some point Lufthansa decided they'd had enough and queried Tower as to whether the Lear in front of them could pick it up a bit. The IP in the left seat had a good chuckle and told me I taxied like his grandmother. Lucky for me, the Euros are pretty civilized when it comes to ATC. If that had happened at JFK, I have a feeling they'd have found a nice quiet place on the airport for us to wait as the conga line went by!

 

ATC at Hurn were pretty good with me, but then it might've been because either they had me orbiting over a sewage treatment plant for over 20 minutes one time, or that if we had special guests at the club-house I could beat them to it, and finish off all cake before they got a look-in. My taxi speeds were typically at 15kts, though as I say I'd take it to almost 30 if anything big was waiting on me, usually Ryanair. Yup the running pace thing was from an instructor and with small single engine piston-prop with nothing fancy fitted in the 'pit.

[Lenux]

I wonder if it's possible to have the " metric " system ( kmh instead knots )