In the real world, Cedrik, a real Australian pilot is being prosecuted for ‘low flying’. My understanding is that the pilot is the pilot to whom the CASA person I quoted earlier was referring when he said to a Senate Committee: “[T]hat’s not what the instruments on his aircraft say. They say he was at 125 feet.”
I confidently predict that the pilot isn’t you, Cedrik, because rather than carrying on like a petulant child as you are in the threads I’ve started, you’d be under constant stress, having sleepless nights and spending large to defend yourself against an allegation you consider to be untrue. Maybe you’d end up like Glen Buckley: a heart attack and penniless as a consequence of what’s been done to you. Pray your turn never comes.
Whilst I am well aware that there are always at least three sides to every story, in the case of “what aircraft instruments say” there are some unassailable facts about the potential delta between “what they say” and a thing called “the truth”. That’s because there are tolerances and margins of error in every gizmo ever fitted to any aircraft, tolerances and margins of error in every gizmo ever used in air traffic control and surveillance and tolerances and margins of error in every forecast ever produced by any met forecaster. And all of that should be BAK.
(Chronic Snoozer nailed it by nominating the RADALT as the most accurate gizmo to measure the distance between a point on an aircraft’s airframe and the ground or water. These days I think the margin of error in RADALTs is specified in centimetres. But why do some aircraft have RADALTs? Precisely because the other gizmos aren’t precise enough when an aircraft is getting close to the ground in zero or bad viz.)
Out of many examples, a prosecution for ‘drink driving’ recently collapsed in the ACT because a legislated period of time was measured by reference to a clock on the wall of a hospital. No evidence was led to prove, to the requisite standard, the accuracy of the clock. When ‘speed cameras’ first came out, many prosecutions for speeding collapsed because no evidence was led to show the serviceability and accuracy of the ‘camera’.
What a speed camera “says”, what a breathalyser “says” and what a clock “says” are all just hearsay. (There are now laws to the effect that provided evidence is led to show a ‘speed camera’ and ‘breathalyser’ were serviceable and ‘in calibration’, the reading is taken to be the actual speed or BAL unless evidence is led to disprove the reading.) And what an altimeter “says” and what a Mode C transponder “says” and what a TAAATS screen “says” are all just hearsay.
Of course we all rely and make decisions on the basis of what these things “say” and it is possible – and necessary – to show the accuracy of “what aircraft aircraft instruments say” if you want to convict someone on the basis of what those aircraft instruments “say”. But simple BAK (such as that provided to us by swh and RedwireBluewire and others in this thread) demonstrates that – depending on what the “instruments” are – there can be hundreds of feet of accumulated difference between the truth and what those instruments “say”. Not a few feet of difference. Not a few tens of feet difference.
As the CASA person who made the statement in front of the Senate Committee wasn’t in the aircraft with the pilot, the CASA person can’t be referring to the aircraft’s altimeter or the display on the aircraft’s transponder. As the pilot involved denies the allegation, I can only assume - reasonably I suggest – that the pilot doesn’t reckon the altimeter was "saying" 125’. I can only assume – reasonably I suggest – that the CASA person was referring to some gizmo on the ground relying on Mode C transponder / ADS-B data.
Acceptable VFR altimeter accuracy when tested against a known accurate QNH and known elevation below 3,300’? +/- 100’. It’s in AIP.
Maximum allowable delta indicated altitude and Mode C data? +/- 125’ (and there’s a margin of error in the Mode C data itself). I’m confident there’s an equivalent of FAR Part 43, Appendix E in Australia’s rules.
And then take off and fly with the altimeter set to forecast Area QNH, which is representative to within +/- 5 hPa of any actual QNH of any location within that Area. The BOM says so.
And what does TAAATS use as the QNH reference to generate the Mode C altitude to display for a particular aircraft?
A pilot diligently nailing 500’ indicated on an altimeter serviceable for VFR and set to Area QNH could, on the published tolerances, be anywhere between about 250’ and 750’ AMSL in fact and squawking a 125’ delta from any number in that range.
There is also the possibility of eyewitnesses. But laypeople are notoriously unreliable in estimating heights and distances of aircraft. When the city of Gunghalin was originally built in the ACT, lots of new residents complained about low flying aircraft. Air Services literally went into people’s lounge rooms with equipment to show that the aircraft (on SIDs out of 35) were, based on the transponder data, at least 2,500’ AGL by the time they were overhead. (I think the data usually showed around 5,000’ AMSL and, taking into account tolerances and the ground level, that meant the aircraft were at least 2,500’ AGL.) The Facebook page for the Brisbane airport noise complaint group includes people complaining about aircraft “shifting gears” above their homes.
To most laypeople, just about every ‘little aircraft’ is a Cessna. But as we know, e.g. a C152 isn’t the same size as e.g. a C208. And when someone sees something that they think is small but is in fact much larger, it appears much closer.
For everyone who thinks this is all trivia and hypothetical, pray you don't come to the attention of someone who wants to hang you on the basis of "what your aircraft instruments say".
Last edited by Clinton McKenzie; 19th Apr 2022 at 02:29.