Part 25 altimeter accuracy
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Joined: Feb 2000
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From: UK
Part 25 altimeter accuracy
I just got asked an interesting question by a curious meteorologist, haven't a clue of the answer, but hopefully somebody here might know?
As we probably all know, altimeters on non RVSM compliant part 25 aeroplanes have to be accurate to ±30ft per 100kts.
Question is - what's that 100kts in? Part 25 just says "speed". I'm guessing is that it is either TAS or IAS/EAS, but I'm really not sure which, and the difference could be worth 50% in some circumstances.
Anybody know? Bonus points for a document reference anywhere.
G
As we probably all know, altimeters on non RVSM compliant part 25 aeroplanes have to be accurate to ±30ft per 100kts.
Question is - what's that 100kts in? Part 25 just says "speed". I'm guessing is that it is either TAS or IAS/EAS, but I'm really not sure which, and the difference could be worth 50% in some circumstances.
Anybody know? Bonus points for a document reference anywhere.
G
Joined: Oct 2006
Aviation Qualifications: A&P
Posts: 1,347
Likes: 271
From: USA
I just got asked an interesting question by a curious meteorologist, haven't a clue of the answer, but hopefully somebody here might know?
As we probably all know, altimeters on non RVSM compliant part 25 aeroplanes have to be accurate to ±30ft per 100kts.
Question is - what's that 100kts in? Part 25 just says "speed". I'm guessing is that it is either TAS or IAS/EAS, but I'm really not sure which, and the difference could be worth 50% in some circumstances.
Anybody know? Bonus points for a document reference anywhere.
G
As we probably all know, altimeters on non RVSM compliant part 25 aeroplanes have to be accurate to ±30ft per 100kts.
Question is - what's that 100kts in? Part 25 just says "speed". I'm guessing is that it is either TAS or IAS/EAS, but I'm really not sure which, and the difference could be worth 50% in some circumstances.
Anybody know? Bonus points for a document reference anywhere.
G
Thread Starter
Moderator
Joined: Feb 2000
Aviation Qualifications: CPL
Posts: 14,480
Likes: 178
From: UK
It is, CS25.1325(e), which I'm aware of...
Whilst it shows a range, it does only show "speed".
G
(e) Each system must be designed and
installed so that the error in indicated pressure
altitude, at sea-level, with a standard atmosphere,
excluding instrument calibration error, does not
result in an error of more than ±9 m (±30 ft) per
185 km/hr (100 knots) speed for the appropriate
configuration in the speed range between 1·23
VSR0 with wing-flaps extended and 1·7 VSR1 with
wing-flaps retracted. However, the error need not
be less than ±9 m (±30 ft).
installed so that the error in indicated pressure
altitude, at sea-level, with a standard atmosphere,
excluding instrument calibration error, does not
result in an error of more than ±9 m (±30 ft) per
185 km/hr (100 knots) speed for the appropriate
configuration in the speed range between 1·23
VSR0 with wing-flaps extended and 1·7 VSR1 with
wing-flaps retracted. However, the error need not
be less than ±9 m (±30 ft).
G
Joined: Mar 2002
Aviation Qualifications: CPL
Posts: 3,385
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From: near an aeroplane
CS25.103 defines Vsr as:
To me that suggests that it should be CAS that is used in the altimeter specification.
The reference stall speed VSR is a calibrated airspeed defined by the applicant.
Joined: Oct 2006
Aviation Qualifications: A&P
Posts: 1,347
Likes: 271
From: USA
It was my understanding V-speeds were a defined value?
Page 149 of the link shows the math for the Part 23 version of the accuracy check. Perhaps it provides the answer?
Page 149 of the link shows the math for the Part 23 version of the accuracy check. Perhaps it provides the answer?
