True Air Speed fluctuations

[Schllc]

I’ve been flying more long trips lately and have noticed some pretty large fluctuations in my true airspeed that I don’t really understand. 
I establish myself at an altitude, lean out and the. Watch my true airspeed vacillate from 205knts to 193knts. There is no attributable difference in outside air temps, wind direction, fuel flow or rpm’s. 
Engine is running like a top, temps are nice and cool.  
This occurs wether running lop or rop. 
The vacillations are not rapid, I am not porpoising with altitude. 
I think I’ve always noticed different speeds at same power settings but usually attributed it to density and weight, but monitoring closely and seeing such a large delta makes very curious. 
Has anyone else seen this?

Last note, it occurs at any altitude I fly from 10-18k. 

[N201MKTurbo]

What time period is the vacillation? If it is fast you may have water in your pitot system, if it is slow, you are probably just experiencing rising and falling air masses. 

[A64Pilot]

And or a phugoid, which is normal for almost all GA aircraft. but watch your GPS speed to see if it’s changing about the same rate as the true, if it is,then it’s real, if not maybe not.

A phugoid really becomes noticeable in real smooth air. hands off flying, hands on we compensate without knowing and many don’t know what a phugoid is

https://en.wikipedia.org/wiki/Phugoid

Don’t read too much into the accident part of this, a phugoid is a normal thing in most GA aircraft and is at most sometimes an annoyance 

On edit, the phugoid I’m speaking of is a long period slow thing,  just a very slow climb and descent so slow that a passenger wouldn't notice.

Edited July 13, 2021 by A64Pilot

[larryb]

Probably up and down drafts. I see the same thing. Look closely at the AI and you might see slight differences in attitude. I also notice slightly higher engine temps when AI is higher and speed lower.

[GeeBee]

It usually happens after dinner is served and everyone is going to the bathroom in the back. The flight attendants complain it is too cold too, because they are no longer working the ovens.  Seriously, you're flying at a constant pressure, not a constant altitude.

 

 

[carusoam]

What instrument are you using to determine TAS?

Is it fully automatic with temp inputs and all?

Very often around here… up drafts and down drafts while using the AP… don’t get noticed… except for the change in airspeed that occurs…

We don’t get a notice each time the AP adjusts trim…

PP thoughts only…

Best regards,

-a-

[Schllc]

3 hours ago, GeeBee said:

It usually happens after dinner is served and everyone is going to the bathroom in the back. The flight attendants complain it is too cold too, because they are no longer working the ovens.  Seriously, you're flying at a constant pressure, not a constant altitude.

 

 

Yes, that is a valid point, but flying across a frontal boundary in relatively stable temperatures, shouldn’t go up and down in an almost cyclical fashion. It’s over more than a few minutes. Too slow to see occurring just very slow increases and decreases. 
I did not think to look at the pitch angle. 

[GeeBee]

Think of the surface of the water in an ocean as a single pressure plane. Now does the boat go up and down relative to a fixed point in space? Some undulations are large and quick like a sheer off a mountain, some are long and slow in an area of high pressure over an ocean. If you had auto thrust, you would not have any change, except you would notice thrust change. As it is, you lock your thrust and. the airplane "rides" the waves in altitude hold.

 

[Marauder]

15 hours ago, Schllc said:

I’ve been flying more long trips lately and have noticed some pretty large fluctuations in my true airspeed that I don’t really understand. 
I establish myself at an altitude, lean out and the. Watch my true airspeed vacillate from 205knts to 193knts. There is no attributable difference in outside air temps, wind direction, fuel flow or rpm’s. 
Engine is running like a top, temps are nice and cool.  
This occurs wether running lop or rop. 
The vacillations are not rapid, I am not porpoising with altitude. 
I think I’ve always noticed different speeds at same power settings but usually attributed it to density and weight, but monitoring closely and seeing such a large delta makes very curious. 
Has anyone else seen this?

Last note, it occurs at any altitude I fly from 10-18k. 

As Larry mentioned above, I suspect you are seeing these variations due to up and down drafts. My guess is you are on autopilot and probably not seeing the minor pitch changes the AP is making. I have seen this a few times flying near higher terrains. Flying an Acclaim I'm betting my +/- 5 knots would be much more dramatic in a plane doing 200+ knots.

[Schllc]

1 hour ago, GeeBee said:

Think of the surface of the water in an ocean as a single pressure plane. Now does the boat go up and down relative to a fixed point in space? Some undulations are large and quick like a sheer off a mountain, some are long and slow in an area of high pressure over an ocean. If you had auto thrust, you would not have any change, except you would notice thrust change. As it is, you lock your thrust and. the airplane "rides" the waves in altitude hold.

 

yes, great analogy.  i understand the principle.  i guess i have just never experienced "waves" of this amplitude because the variations have typically been only a knot or two, never over 10.  or at least i never noticed...

[Schllc]

11 hours ago, carusoam said:

What instrument are you using to determine TAS?

Is it fully automatic with temp inputs and all?

Very often around here… up drafts and down drafts while using the AP… don’t get noticed… except for the change in airspeed that occurs…

We don’t get a notice each time the AP adjusts trim…

PP thoughts only…

Best regards,

-a-

g1000

[Ragsf15e]

Try turning off the autopilot, trimming it really well and then just holding speed (vfr). See if your altitude goes up and down with constant speed…

[jlunseth]

I'd say the fluctuations are on the outside edge of what you might see with updrafts and downdrafts but your aircraft is naturally faster than mine. The pickup in speed can be pronounced when you experience a downdraft, sometimes the aircraft noticeably noses down and you are running down hill. Acclaims being as slick as they are, probably that is what causes it to go over 10 kts. Usually in the vicinity of 5-10 knots, more prevalent some days than others, I have noticed them more while flying out over the Great Plains than anywhere else.

[PT20J]

1 hour ago, Schllc said:

g1000

Is the IAS varying or just the TAS? What’s the GS doing?

If you are holding altitude and the IAS varies it is likely up and down drafts.

If the IAS is steady, it’s some sort of calculation error since the G1000 calculates TAS.

Skip

[Davidv]

I see the same thing and I've suspected it's my pitot system but maybe this is more of a normal occurrence?  I see my GPS speed stay roughly the same while my headwind component changes at about the same time as my TAS.  Of course, there are tiny pitch changes that cause this too but when I see my TAS go from 200-193 in a one minute period it must be instrument related.

[Ragsf15e]

Not exactly sure where you’re flying but I fly a G1000 PA46T around the northwest.  Even without really strong winds, I’ve seen +\-15 kts at altitude from mtn wave at 20,000’ish and a very long way from the cascades…. The rockies and sierras will do that too and it goes a long way down range.  These kinds of fluctuations don’t take mtn wave sigmets or anything.

[carusoam]

One thing for sure….

Long smooth mountain waves don’t occur every day…

So…. Does this speed variation occur often? Just this once? Or only when flying in the shadow of mountains?

 

Hmmm….

What occurs when you pass a cold/warm front?  As the OAT changes… does this have a large affect on the TAS?

One thing for sure…. When warm air is forced to climb over a cold air mass…. You can really catch the up elevator that way… 

 

Great complex subject…

Best regards,

-a-

[A64Pilot]

Take your airplane in dead smooth air with it trimmed perfectly and just let it fly hands off, it will develop a Phugoid, meaning it will slowly climb and descend and speed up and slow down, all on its own 

To Certify an Airplane for Canada it has to be demonstrated to fly some amount of time in level un accelerated flight hands off, it may be two minutes I’m not sure. FAA doesn’t have this requirement.

So when I was doing the Certification test flights to Certify the S2R-H80 for Canada I adjusted the aileron servo tabs as precise as I could and very early one morning went out and let her fly for quite some time hands off, and it slowly started accelerating and slowing, just a few MPH it wasn’t much and it wasn’t divergent. I’d never noticed this before and called my Mentor who was the Flight test DER and he laughed and said it’s a Phugoid, pretty much all aircraft will do it, but most never notice it as it’s so long period and such a slight thing that pilots dampen it out and never know it.

‘I am not saying he’s in a Phugoid, but it’s possible if he or the autopilot wasn’t adjusting to maintain altitude.

But if true AS is changing and ground speed isn’t, and especially if indicated isn’t, then it’s an instrument error, but if all three change, then of course it’s real.

Edited July 15, 2021 by A64Pilot

[Schllc]

Update....

same route flown this weekend. Same gross weights, same altitudes (1000 ft diff)

no tas fluctuations noticeable, other than typical 1-2 knots. 
 

ps, it’s south Florida to south Louisiana, no mountains that I know of around there....

[Ragsf15e]

12 hours ago, Schllc said:

Update....

same route flown this weekend. Same gross weights, same altitudes (1000 ft diff)

no tas fluctuations noticeable, other than typical 1-2 knots. 
 

ps, it’s south Florida to south Louisiana, no mountains that I know of around there....

Warm air rising updrafts ?  Time of day difference between the flights?

[Schllc]

Nothing discernible on the oat.  Did have a consistent tail wind instead of a consistent headwind. Earlier in the morning by a few hours, one way, about the same the other way.