Can you elaborate on this? The person doing the test hooked up their equipment to both the pitot tubes and static ports.

They do this to avoid breaking mechanical airspeed instruments. If you didn't take pitot pressure down with static, the airspeed indicator would wind up well past max reading and get damaged.

Modern electronic sensors would survive this, however.


That is about as accurate as the winds aloft method can be. The most accurate method is the GPS course method.


If you get SAT, that is what you compare to the sounding temps.


Cross check is good.

If the temperature is more than 2 C apart, that's quite a bit.

You can also compare KTAS readouts.

My left ADC indicates about 2 knots faster than my right, so I joke that the copilot arrives a little after I do.

Mike C.

Chris,

Your plane is fast as hell. 374 or 368 doesn't matter. Don't mess with it at all!!!!

I seriously don't know if I would have gotten rid of my 501sp with speeds like that. 30-40 pts TAS is huge. That's damn close to the stallion or Sierra Williams speeds without the costs associated. Really cooking. Really cool. Hope you are loving it!

That's so large an increase that it falls well outside normal variation.

The amount of power goes up as the cube of airspeed. If we go from 335 to 370 knots, that's a 10% increase in speed, which requires 35% more power. That's not believable.

That could be explained by a 21% drag reduction. That is also not believable.

So it is hard to understand where the difference comes from.

Mike C.

Power goes as the cube of speed. However, thrust/drag go as the square of the speed. So a 10% increase in speed either requires a 21% increase in thrust or a similar 21% reduction in drag. Not very likely. Points to an instrumentation error.

I'd be curious to know what that is.

Can't be anything structural, windows, engines, landing gear, door, avionics, HVAC, etc.

So what's left?

Mike C.

I'd be curious to know what that is.

Can't be anything structural, windows, engines, landing gear, door, avionics, HVAC, etc.

So what's left?

Mike C.

Mike, is TAT on the Garmin equal to RAT in the Citation manual?

I think we can discard the predicted temps aloft as those could have easily been off by 10 degrees.

-29C RAT in the 501 manual corresponds to about -42C OAT. So let's split the difference between that and your calculation of -46C and say the OAT was -44C. That's ISA+10.

ISA+10 will increase TAS by about 6kts for the same IAS. However, this would only be a factor if we think the ADC was using the wrong temp value somehow. Long shot, but some issue with F/C conversion when Chris selected F on his TAT display?

The 501 manual at 9,500 lbs, FL350, ISA+10 shows a max N1 of 100, TAS 341, and fuel flow of 815 lbs/hr.

So if Chris was doing 1000 lbs/hr, that is 23% more fuel flow, which (interpolating from the charts) gives about 15% higher TAS (although in theory it should be about 11%). Seems to me that N1 would be higher than 102% in that situation (and the manual only goes up to 100% at that temp/alt - is that a limitation?) Have to wonder what the engine temps would be in that case.

This theory makes a lot of sense. Or if not mountain wave, perhaps some type of convective lift.

Chris didn't mention how long he was at this speed. Also he indicated that he previously had not flown this fast. So it does seem to be an anomaly.

Probably the best way to check his TAS indication is to fly another plane with a known good TAS readout parallel and as close to his plane as ATC allows at FL350.

Another idea is he could fly a track, turn 180 and fly the other way, and then fly the same way again as a control. Or fly a circle. One of these methods could determine winds aloft and that could be added/subtracted from GPS groundspeed to give TAS. Of course this would not account for mountain wave if that is present.

Yes, TAT (total air temperature) is RAT (ram air temperature).

Likewise, SAT (static air temperature) is OAT (outside air temperature).

TAT and SAT are modern terms that more precisely articulate the meaning.


What predictive temperature as you referring to?

The upper air data is measured temperature by a weather balloon, not predicted.

Given it was observed 40 minutes after and about 50 nm away, the temperature of the flight was very likely quite close to the measured value. That doesn't change that much in a stable air mass.

It measured -52.5 C SAT. At 210 KIAS, that's a 17 C ram air rise, so TAT was -35.5 C. His measurement was -29.4 C, about 6 C off, which seems sloppier than a quality temperature probe should be.

ISA at FL350 is -54 C, so it was ISA + 1.5 C, not too far from ISA.


That's a lot more than book, but he was a lot closer to ISA, too.

I am at a loss to explain the numbers.

It almost sounds as if there is an STC to put a -4 engine (from a 550) on a 501 and that's what he has. He would have higher fuel flow, higher speeds, and not exceed ITT or N2 that way.

Mike C.

I think there are a couple 501's or 500's with -4's out there. Some type of Sierra mod? Maybe Chris has one and doesn't know it.

Still looking into the mountain wave theory. It makes a lot of sense based on the strong westerly winds and his position downwind from the Rockies. I notice his pitch attitude in the photo is just barely nose up. I wonder what it usually is? If it's normally a couple degrees up at FL350 then this is a clear indication the autopilot is compensating for an updraft.

Line up boys and poor the coals to them, see who gets to the destination first….
Bicycles, motorcycles, boats, cars, and now jets….

Peace,
Don

Might be worth checking the dataplate. You never know!


I looked at the groundspeed on this flight and they are steady over long distances. That doesn't happen with wave.

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At 210 KIAS, it won't be much above 0. If you look at my PFD pic, I'm at 256 KIAS and below zero attitude at FL330.


Steady speeds suggest this wasn't wave or updraft.

Mike C.

See attached STC for installing JT15D-4 on 500 series.

I would be funny if he has -4 engines and doesn't know it.

Mike C.

See attached STC for installing JT15D-4 on 500 series.

I would be funny if he has -4 engines and doesn't know it.

Mike C.