What is the allowable altitude difference reading between two altimeters while flying inside RVSM airspace?
On the ground during the preflight check, the altimeters must be within 75 feet of each other.
In-flight the altimeters must be within 200 feet of each other.
I have been flying the B777 for about 8 years and I have never seen the altimeters disagree by more than about 10 feet, either on the ground or in flight.
I couldn't find an FAA regulation, but AC 91-85B Authorization of Aircraft and Operators for Flight in Reduced Vertical Separation Minimum (RVSM) Airspace Document Information page B-4 says 200ft:
At cruise FL, the two primary altimeters should agree within 200 ft (60 m) or a lesser value if specified in the aircraft operating manual. (Failure to meet this condition will require that the altimetry system be reported as defective and notified to ATC.) Note the difference between the primary and standby altimeters for use in contingency situations.
It also says on page B-3 that before takeoff both altimeters must be within 75ft of field elevation:
The aircraft altimeters should be set to the barometric pressure for local altimeter setting (QNH) and should display a known elevation (e.g., field elevation) within the limits specified in aircraft operating manuals. The difference between the known elevation and the elevation displayed on the altimeters should not exceed 75 ft
Piling on to an earlier answer, but with some more tangible references. Noting that your post doesn't specify which country you're flying in, there does seem to be an international standard (defined by ICAO). I highly recommend using this resource to help understand several relevant regulations.
Starting with the FAA...
The FAA regulation is found in Part 91, Appendix G. The advisory circular referenced above is the most accessible way to process RVSM Requirements and is updated to the newest RVSM law (from Jan '19).
From the advisory circular:
A.5 Altimetry System Performance.
A.5.1 General. The statistical performance statements of International Civil Aviation Organization (ICAO) Doc 9574, Manual on a 300 m (1,000 ft) Vertical Separation Minimum Between FL 290 and FL 410 Inclusive, for a population of aircraft are translated into airworthiness standards by assessment of the characteristics of ASE and A-9 1/29/19 AC 91-85B Appendix A altitude control. The following standards differ in some respects from that document, but they are consistent with the requirements of RVSM and in accordance with 14 CFR part 91 appendix G, section 2.
Here, you see FAA concordance in spirit and substance with ICAO. If you're interested in the nuanced details which defines a valid altimetry system for certification, you should read this entire section. In short, it defines a max standard deviation of Mean Altimetry Symmetry Error (how much the altimeters differ) at 80ft, and 3 standard deviations at 200 ft. This is then used to derive the inflight guidance found in Appendix B:
- At cruise FL, the two primary altimeters should agree within 200 ft (60 m) or a lesser value if specified in the aircraft operating manual. (Failure to meet this condition will require that the altimetry system be reported as defective and notified to ATC.) Note the difference between the primary and standby altimeters for use in contingency situations.
Moving to ICAO, you can see ICAO Verbiage here:
In respect of aeroplanes for which the characteristics of the airframe and altimetry system fit are unique and so cannot be classified as belonging to a group of aeroplanes encompassed by paragraph 1, the height-keeping performance capability shall be such that the components of the TVE of the aeroplane have the following characteristics:
a) the ASE [Altimetry Symmetry Error] of the aeroplane shall not exceed 60 m (200 ft) in magnitude under all flight conditions; and
b) the differences between the cleared flight level and the indicated pressure altitude actually flown shall be symmetric about a mean of 0 m, with a standard deviation no greater than 13.3 m (43.7 ft), and in addition, the decrease in the frequency of differences with increasing difference magnitude shall be at least exponential.
Finally, it's interesting looking at the European Aviation Safety Agency (EASA) SPA Verbiage:
two primary altitude measurement systems. A cross-check between the primary altimeters should be made. A minimum of two will need to agree within ±60 m (±200 ft). Failure to meet this condition will require that the altimetry system be reported as defective and air traffic control (ATC) notified;