Woke up in the morning to a sms from an ATR 72 pilot asking about the ability of the ATR 72 to fly a GPS approach to minimums.
The question, and my search through my files got me a bit confused. There was no clarity on the viability of the on board HT 1000 to support GPS approach. So I had to head online.
It was a crazy search, but search I did, nevertheless! I found online an Airworthiness Approval document from the UK CAA, which specifies under limitations:
“The GNSS HT1000 must not be used for GPS approaches.”
This question has clearly caught me off-guard, and I’m now trying to understand what makes a non-WAAS GPS approach feasible. And another thing bothering me is whether this approval is only for the specific aicraft: : G-BXTN.
The Airworthiness Approval may be read here:
http://www.caa.co.uk/aandocs/26265/26265000000.pdf
If you can throw some light, it’d be great!
UPDATE: The fact that the unit is certified for RNAV 1 opertions, and approaches equire RNP 0.3 or 0.5 automatically disqualifies the aircraft from shooting GPS approaches with an ATR 72-200/500
The Flying Engineer 
Ha Ha Ha…. you did all that……. “This equipment i certifeid for use as a VFR and IFR supplemental navigation means , enroute and in terminal area upto the IAF”…
didnt even have to google is…. or ask JAAR or FAA….. found it in the FCOM
but also……. in APR 08 FAA and JAA approved the systems capability to do GNSS approaches… in single and dual installaton both……
AIRPLANE FLIGHT MANUAL.
EASA
APPROVED
JUL 08
1 — GENERAL
The Honeywell/Trimble GNSS HT1000 :
– complies withTSOC129andTSOC115B,
– is installed in compliance with, AC20–130A and AC20–138 for navigation use
– is installed in compliance withFAAAC20–129 for the advisoryBaro–VNAVuse
– has been demonstrated to meet primary means of navigation in
oceanic/remoteareasinaccordancewithFAANotice8110–60orequivalentin
dualHT1000configuration.
– has been demonstrated to meet the En–route continental B–RNAV
requirements of JAA TGL no2,REV1 or equivalent in single or dual HT1000
configuration.
– has beendemonstrated tomeet theP–RNAVrequirements of JAATGLno10.
– has been demonstrated to meet the RNAV (GNSS) non–precision approach
requirement ofAC20–138 andRNPAPCHspecification both in single or dual
HT1000configuration.
2 — LIMITATIONS
Compliance with the above regulations does not constitute an operational
approval/authorization to conduct operations. Aircraft operators must apply
to their Authority for such an approval/authorization.
– The HT1000pilot’s guide must beavailableonboard.
– The system must operate with HT1000–060 software version or any later
approved version.
– The approval of the system is based on the assumption that the navigation
database has beenvalidated for intended use.
– This system is approved in dual configuration for use for oceanic and remote
operations as aprimary navigationmeans.
– InsingleHT1000configuration, thesystemisapprovedforuseforoceanicand
remote operationswhenonly onelong range navigation system is required.
– ThissystemisapprovedforRNAVEn–routecontinental(B–RNAV)andRNAV
terminal area operations likeRNAVSID/STAR(P–RNAV).
NOTE : This system is approved in any of the following configurations: AP
coupling, FD only or raw data (HSI information).
– This system is also approved as a supplemental navigation means for
conventional En–route continental operations, conventional terminal
operations (SID/STAR) and conventional non–precision approach (e.g.
NDB, VOR/DME, LOC,…) provided :
S Approved navigation equipments, other than GNSS, required for the
route to be flown are installed and operational.
S When the system is used to fly non–precision approach not promulgated
asRNAV(GNSS) approach orwhen procedure coordinates
(SID/STAR, Approach) cannot be guaranteed asWGS84, raw data
(conventional navigation information) are displayed to verify the correct
RNAV(GNSS) guidance.
– Both single and DualHT1000 configurations are approved forRNAV(GNSS)
non–precision approach provided:
SThe crew respects the published MDA (without VNAV credit)
SThe published approach procedure is referenced to WGS84 coordinates
or equivalent.
SBefore starting the approach, crew checks that for dual configuration at
least one GNSS is operating without DGR annunciation or for single
configuration GNSS is operating without DGR annunciation.
SAPP annunciation must be displayed in cyan on EHSI for final approach.
NOTE : Pilots intending to conduct an RNP APCH procedure must fly the
full leg starting from IAF otherwise the system will not switch to
APP mode (RNP and EHSI scale will remain at 1NM)
SRNAV(GNSS)non–precision approaches are performed only if a
non–GNSS approachprocedure is availableat destinationor at alternate
destination.
SRNAV(GNSS)non–precision approaches must beaborted incaseofDGR
annunciationonEHSIand/orUNABLERNPmessageonMCDU
affecting theRNAV(GNSS)used for guidance.
SApproved navigation equipments, other than RNAV(GNSS), required for
the approach to be flown (at destination and at any required alternate
airport) are installed and operational.
NOTE : ILS, LOC, LOC BC, LDA, SDF and MLS approaches are not
covered.
– The RNAV (GNSS) system is approved for use as advisory Baro–VNAV
system.
NOTE : VDEV function must be permanently cross–checked by
conventional means (primary altimeters displays)
– If GNSS must be used in oceanic/remote area, B–RNAV (if DME are not
available), P–RNAV or for approach phases, the availability of the GPS
integrity (RAIM or FDE functions) must be checked by the operator using
prediction tool available in the GNSS during the pre–flight planning phase or
any other approved tool
3 — PROCEDURES FOLLOWING FAILURE
– DUAL GNSS :
SFor RNAV (GNSS) non–precision approach : in the event of both DGR
alarm illuminations or if “VERIFY POSITION” message occurs, perform
a go around unless suitable visual reference is available.
SIn case of loss of navigation or navigation degradation leading to the loss
of the required navigation performance crewmust inform ATC and revert to
alternate navigation means.
– SINGLE GNSS :
SFor RNAV (GNSS) non–precision approach, in the event of DGR alarm
illumination or if “UNABLE RNP” message occurs, perform a go around
unless suitable visual reference is available.
SIn case of loss of navigation or navigation degradation leading to the loss
of the required navigation performance crewmust inform ATC and revert to
alternate navigation means.