Diagnose this ...

[old man emu]

Can you diagnose this situation and explain it's cause?

 

It's Sunday morning and you and your "significant other" decide to take a flight to get a $100 hamburger at a beach side airport. You check the weather and find that your intended flight path is dominated by a strong high pressure cell of up to 1024 hPa and winds are light and variable below 5000. Temperature is in the low 20's; humidity in the 40's and dew point is below 10C.

 

You take off and set for cruise, checking that you have "Carby - Cold; Friction Nut - firm; fuel On and sufficient quantity; oil pressure and temp "in the green"; RPM set to 75% cruise power. The aircraft is trimmed nicely. The needle of the VSI isn't moving away from Zero. The arrow of the ASI points directly at your desired value.  God's in His heaven and all's well with the world.

 

After about half an hour you cross-check your flight plan, map and GPS and find that your ground speed is a few knots less than expected, and you are behind ETA, but dead on track.

 

What actions do you take to diagnose why you are slow and late, and what could be the cause?

[Old Koreelah]

Left the wheel chocks attached to the U/C?

[RFguy]

headwind ? Pitot / static system ?  incorrect IAS-TAS conversion for temp,hum,press ? 

[old man emu]

Very interesting RFguy, but no

Read the scenario again.

[rgmwa]

Winds have picked up and you've now got a headwind?

[M61A1]

I want to know why it took half an hour to realise there's an issue.

Generally I'd say the wind is the cause, remember they are "light and variable", or do you want us to tell you why the wind is variable?

[Kiwi]

My wife took her handbag and am way over MTOW cause of it. 

[M61A1]

32 minutes ago, Kiwi said:

My wife took her handbag and am way over MTOW cause of it. 

Yes, I can sympathise....

[old man emu]

The weather system is a stationery High Pressure one that hasn't moved for a couple of days, and is not forecast to move for a few more days.

1 hour ago, M61A1 said:

do you want us to tell you why the wind is variable?

No. Note the tag to the thread - "Systems Knowledge"

 

Nobody has yet said what actions they would take to diagnose the cause.

[old man emu]

1 hour ago, M61A1 said:

I want to know why it took half an hour to realise there's an issue.

'Cause it's a lovely day and you've flown this route umpteen times. You have been sitting there enjoying the view TIBMIN.

[Yenn]

I think the answer lies in the fact that you are flying to a beahside destination and left after mid day in light and variable conditions. I leave you to work out what my thinking is and no doubt OME will correct me if I am incorrect with my thoughts.

[Ryanm]

TAS correct for given RPM and conditions?

[old man emu]

You can't answer the question until you have checked the aircraft's SYSTEMS.

[RFguy]

so ho wcome my original answer
"headwind ? Pitot / static system ?  incorrect IAS-TAS conversion for temp,hum,press ? "

 

is incorrect ???

[M61A1]

Could be a leak in your static system making your ASI overread. (If the leak in inside the aircraft and cabin pressure is lower than actual static pressure, perhaps even a leaking ASI case) Or a poorly located static port.

[Thruster88]

I feel you are suggesting there has been a slight loss of power, does the aircraft have a Constant Speed propeller to complicate the diagnosis 🤔😂🙄

[M61A1]

1 minute ago, Thruster88 said:

I feel you are suggesting there has been a slight loss of power, does the aircraft have a Constant Speed propeller to complicate the diagnosis 🤔😂🙄

I got the impression that he was looking at the greater than expected difference between the ASI and groundspeed.

[rgmwa]

You are flying lower than the altimeter is reading so your indicated airspeed is higher than than you expect you TAS to be for your  planned altitude so you are slower over the ground than you anticipated.

[Thruster88]

In the real world the pilot would assume that there is a head wind and no diagnosis would happen. 

[old man emu]

2 hours ago, Thruster88 said:

I feel you are suggesting there has been a slight loss of power, does the aircraft have a Constant Speed propeller to complicate the diagnosis 🤔😂🙄

No. Fixed pitch prop.

I think that Thruster might win the prize. Remember, you note that track made good is right on planned track.

 

3 hours ago, RFguy said:

so how come my original answer
"headwind ? Pitot / static system ?  incorrect IAS-TAS conversion for temp,hum,press ? " is incorrect ???

The scenario does not provide data for that. Read the scenario again.

 

Nobody has listed the actions they would take to diagnose the situation. That's where the answer lies.

[kgwilson]

I don't know why it should take half an hour to work out your ground speed is less than your IAS (converted to TAS). I'd check this as soon as cruising altitude is reached & adjust the throttle to get the GS up as would seem there is a small head wind. Otherwise I'd accept the longer ETA and conserve fuel by continuing at 75% power or adjust altitude to see if there is any change. Even though the pressure is high, unless you are in the middle of the high the air will be moving in an anticlockwise direction to areas of lower pressure and also downwards. This effect will be reduced with altitude.

[Jerry_Atrick]

Clearly, this is a tech failure.. So as the GPS is nothing more than a fancy computer, diagnostics would start with recyling it (switching it on and off - not turfing it for another) and if that didn't correct the GS, tap it on the screen a few times where the GS is displayed.

 

If it was still showing a low GS, then I would remain suspicious of the unit, but just to be sure, check the ASI isn't busted by taping it on the glass (but not too hard).. Seems OK..

 

Of course, it could be alternate static air is open and should be closed, but as we are so diligent in our pre-flight checks, that would never happen... Maybe - as in some a/c such as the Warrior; drain the static lines as well... Nah.. we did that during pre-flight.

 

Once done, it would be a precuationary landing and as per the user guide, calling the GPS authorised service representative and wait for a repair guy to take a look...

Edited October 14, 2020 by Jerry_Atrick

[old man emu]

I'm starting to wonder if allowing pilots to maintain their aircraft is a good idea. I'm not seeing any listing of the actions being taken to diagnose the problem. So far only two people have hinted at aircraft systems and one of those persons is wrong.

 

As for the half hour to notice that achieved ground speed was not as planned, I said that the pilot was flying a familiar route TIBMIN and enjoying the view.

[rgmwa]

Radio a request for an area QNH and check the altimeter setting to see if you're flying at the altitude you think you are.

[Student Pilot]

Why did it take half an hour? As soon as you reached cruising height you would have checked GS on the GPS. Al this angst over a couple of knots or 250ml an hour difference in fuel flow seems unwarranted.

Not as if you have a company on your back to minimize fuel flow and economy for maximum profits. The object is to go for a jaunt and enjoy the flight. If you constantly scanning graphs and charts to get optimum fuel flow and listening out for possible catastrophic failure about to appear your beloved pax might start to worry about your imminent demise instead of salivating over the seaside menu. 

You must be a joy to fly with 😁