Turbulence speed Questions
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Turbulence speed Questions
Hi guys,
If you are flying tru some clouds/turbulence, would you fly according to your Va / Vra or the Max Turbulence Speed? As what I know Va is less then Max Turbulence. Speed and practical wise.. the ride will be smoother in Va as what I've experienced.
And ya the Hydroplanning speed formula = 9 (square root) of the tyre PSI.. which tyre should we take.. the nose wheel or the main wheel? Because clearly some airplane has difference PSI setting for each tyres.
Hope you can share some light here.
Cheers mate
If you are flying tru some clouds/turbulence, would you fly according to your Va / Vra or the Max Turbulence Speed? As what I know Va is less then Max Turbulence. Speed and practical wise.. the ride will be smoother in Va as what I've experienced.
And ya the Hydroplanning speed formula = 9 (square root) of the tyre PSI.. which tyre should we take.. the nose wheel or the main wheel? Because clearly some airplane has difference PSI setting for each tyres.
Hope you can share some light here.
Cheers mate
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If you are flying tru some clouds/turbulence, would you fly according to your Va / Vra or the Max Turbulence Speed? As what I know Va is less then Max Turbulence. Speed and practical wise..
And ya the Hydroplanning speed formula = 9 (square root) of the tyre PSI.. which tyre should we take.. the nose wheel or the main wheel? Because clearly some airplane has difference PSI setting for each tyres.
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Flying only through clouds without light to moderate chop is no reason to slow down. We normally don't reduce speed with every "bump" in the sky. There has to be at least sustained chop before reducing to "turbulence penetration speeds" which on the B747 is 290-310kts or M.82 to M.85, whichever is lower. Because of the large airplane inertia, at high altitude at heavy weights, it's important not to get too slow, better to stay at the upper end of the turbulence penetration speeds.
[Va speed is max maneuvering certification speed at which you could safely stall an airplane. During certification, this has to do with forces measured on the elevator. Above Va you may exceed structual limit before reaching a stall].
Hydroplaning speeds would be based on the 16 main wheels, rather than the 2 nose wheels, as directional control at high speed is an aerodynamic function of the rudder. The nose wheels, which have no brakes, have virtually zero steering effectiveness above 80kts on a wet pavement. The B747 nose wheels are identical in size and pressure to the main wheels.
[Va speed is max maneuvering certification speed at which you could safely stall an airplane. During certification, this has to do with forces measured on the elevator. Above Va you may exceed structual limit before reaching a stall].
Hydroplaning speeds would be based on the 16 main wheels, rather than the 2 nose wheels, as directional control at high speed is an aerodynamic function of the rudder. The nose wheels, which have no brakes, have virtually zero steering effectiveness above 80kts on a wet pavement. The B747 nose wheels are identical in size and pressure to the main wheels.
Re. Hydroplaning (Aquaplaning in the UK).
Main wheel tyre pressure, since these are the ones that are going to stop you.
Another important factor is the depth of tread on the tyres. I've seen a lot of tyres with less than 2mm of tread remaining in service. If the water has no where to go then there is more chance for aquaplaning to occur.
Hope this helps.
Glue ball. What a sensible A/C the 747 is from a line maintenance point of view. All 18 wheels the same and interchangable with the same tyre pressure, verses the DC10 with 2 different wheels and 3 different tyre pressures. ( early '80s when I worked on them).
Main wheel tyre pressure, since these are the ones that are going to stop you.
Another important factor is the depth of tread on the tyres. I've seen a lot of tyres with less than 2mm of tread remaining in service. If the water has no where to go then there is more chance for aquaplaning to occur.
Hope this helps.
Glue ball. What a sensible A/C the 747 is from a line maintenance point of view. All 18 wheels the same and interchangable with the same tyre pressure, verses the DC10 with 2 different wheels and 3 different tyre pressures. ( early '80s when I worked on them).
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Thanks for the replies...
So if that's the case, if flying tru severe chops or inside huge CB's, do we slow down to Va or Vra? Since Vra > Va there are chances of structural damage to the airplane before it stalls especially with the severe up and downdraught.
Or is it if we are jsut crusing tru we do a Vra or if we were to alter our heading we are to reduce to Va first before turning?
So if that's the case, if flying tru severe chops or inside huge CB's, do we slow down to Va or Vra? Since Vra > Va there are chances of structural damage to the airplane before it stalls especially with the severe up and downdraught.
Or is it if we are jsut crusing tru we do a Vra or if we were to alter our heading we are to reduce to Va first before turning?
So if that's the case, if flying tru severe chops or inside huge CB's, do we slow down to Va or Vra?
Furthermore, below Vra/tp you may give up a large margin of your stall protection, so you increase the risk of a lower speed stall but despite a change in angle of attack you will hopefully avoid the stall and you can not impart more than a limit load upon the craft-if you do have an accelerated stall as a result of a vertical gust...I feel this may be controversial hence that long explanation I gave in those two links
Va is not a listed speed for operational use. So, in reality you wont compute/estemate it, unless you have the Vs1, and have taken the time to previously estimate it for your plane---further while the gust may not be sharp-edged enough to impart an instantaneous AoA change imparting the limit load--the gust may still force you to stall-therefore stall recovery also becomes a part of the whole mess---lastly the aircraft most likely never encountered such extreme stalls in certification therefore there's no really knowledge of what may happen--despite the load factor being [hopefully] under the limit load
you just have to---in that special case-- of possible,
Extreme Turbulence---Slow Way Way Down!!!
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Pugilistic Animus
Extreme Turbulence---Slow Way Way Down!!!
Maintain Turbulence Penetration speed; B747: 290-310kts or M.82 to M.85, whichever is lower, to avoid low speed buffet/stall.
Glueball
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I said it's controversial---read the LONG justification, for that statement http://www.pprune.org/professional-pilot-training-includes-ground-studies/388963-principles-flight.html---It's not in the manuals, or even [really] in the FARs, and it even goes a little against the advice in HTBJ- which is what your manual says to do- although he comments about today's speeds are pushing too close to the high speed/buffet boundary area.......
I'm talking about ultimate survival, from an extreme circumstance well beyond the protections of Vtp or FAR 25.343...
25.343 gust envelope protections are very comprehensive...but...
.
Way Way WRONG.
Maintain Turbulence Penetration speed; B747: 290-310kts or M.82 to M.85, whichever is lower, to avoid low speed buffet/stall.
Maintain Turbulence Penetration speed; B747: 290-310kts or M.82 to M.85, whichever is lower, to avoid low speed buffet/stall.
I'm talking about ultimate survival, from an extreme circumstance well beyond the protections of Vtp or FAR 25.343...
25.343 gust envelope protections are very comprehensive...but...
Last edited by Pugilistic Animus; 17th Aug 2010 at 17:10.
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High speed buffet worries . . .? B747 MMO is M.92; Turbulence penetration speed is M.82 - M.85. Uncle Boeing says "Do not allow the airspeed to decrease and remain below the turbulent penetration speed as buffet margin will be reduced . . ." [Boeing Flight Crew Training Manual].
Glueball:
I'm not disagreeing with the flight manual, I'm just putting this out there, I think that most Mtp/Vtp also give a rather significant margin above the low speed buffet or structural strength margins I feel that the choice of rough airspeed in conjunction with the turbulence protection is very good,
but there are compromises so that 1. a reasonably economic speed can be flown and 2. that structural weight can be kept down...however, there exist conditions that are so extreme that the FAR 25.343 protectios are overcome and only the physical reality of load factor imparted and operating strength limitations are relevant...no OEM can advise a lower speed than the Mtp/Vtp---- imagine what the lawyers would do with that?...but FAR25.343 has several 'loopholes' and I felt that in case such an extreme situation may be encountered folks would be armed with those facts...I guess it would be upheld under 91.3 ...sorry for the controversy...but I HAD to write this because in the original link I thought that the question itself was misleading and poorly worded and that section is particularly difficult to interpret
I'm not disagreeing with the flight manual, I'm just putting this out there, I think that most Mtp/Vtp also give a rather significant margin above the low speed buffet or structural strength margins I feel that the choice of rough airspeed in conjunction with the turbulence protection is very good,
but there are compromises so that 1. a reasonably economic speed can be flown and 2. that structural weight can be kept down...however, there exist conditions that are so extreme that the FAR 25.343 protectios are overcome and only the physical reality of load factor imparted and operating strength limitations are relevant...no OEM can advise a lower speed than the Mtp/Vtp---- imagine what the lawyers would do with that?...but FAR25.343 has several 'loopholes' and I felt that in case such an extreme situation may be encountered folks would be armed with those facts...I guess it would be upheld under 91.3 ...sorry for the controversy...but I HAD to write this because in the original link I thought that the question itself was misleading and poorly worded and that section is particularly difficult to interpret
