Date: 201910
Location: MAF.Airport, TX
Aircraft: Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class C MAF
ATC / advisory: Tower MAF
Aircraft component: GPWS
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Confusion
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Procedure; Aircraft
Primary problem: Ambiguous

Narrative: It was FO's (First Officer) leg departing from Runway 28 MAF. Weather was VFR with gusty winds out of the west. We departed from 28; and at 500 ft. AGL we received the 'Terrain; Terrain; PULL UP.' We executed the terrain avoidance maneuver until it ceased; which was at 4600 ft. MSL. FO pitched the nose over to start cleaning up the aircraft and the 'Terrain; Terrain; PULL UP' went off again. FO executed the terrain avoidance maneuver until about 5600 ft. MSL. FO leveled off yet again; cleaned up the aircraft; and then flew to our destination with no further problems.Trying to make sense of the situation; at cruise altitude; we reviewed what had happened. We took a closer look at the takeoff data and realized we misinterpreted the station information page noting the code needed for a 28 departure. We interpreted C/28 as being the full length when it needed to be the total distance. In the future the crew will be more diligent as to making sure takeoff data is clear. Also making sure a takeoff/landing report is provided by Operations could also have helped this situation. We should have made sure to have a takeoff/landing report present and reviewed it prior to departure. It seems like these are optional paperwork items but are important. Despite having recently gone through MAF; it is a good idea to review the station notes every departure and arrival. Additive conditions can change so as to have different parts of a station information page become or don't become relevant.


Second narrative: The weather was clear with gusty winds from the west; we elected to takeoff on Runway 28 at MAF. On takeoff roll; the autothrottles would not engage. I checked both FDs were on; and manually set the throttles. The rest of the takeoff roll was normal. Immediately after gear retraction; the GPWS began a Terrain Caution; followed by a Terrain; Pull Up warning. We initiated the avoidance maneuver. The Pull Up warning continued until approximately 4600 ft. MSL; estimated; at which point we pulled throttles back and began to reduce pitch. The Terrain Pull Up warning immediately began again and we resumed the avoidance maneuver again. At approximately 5600 ft. MSL; the warnings stopped. We then reduced power and pitch; cleaned up on speed and proceeded on our way. We were both very confused about what triggered the warning; but executed the procedures anyway. There is no terrain west of MAF; and the GPWS never issued an Obstacle Caution or warning; only Terrain. The altitudes are our best estimate. We were definitely caught off guard and were doing our best to execute the procedures.At cruise; we carefully looked at everything we did. We realized that we had misinterpreted the notes concerning the weight restrictions for the Runway 28 east of Taxiway C and the displaced threshold; to mean that the C intersection was 'full length' for takeoff. When the weight and balance full-length data came back; it reinforced my thought that the data was correct. I assumed the data would flag a runway option we couldn't use. In hindsight; the station notes make perfect sense; but my expectation bias hindered my analysis. I don't believe this would affect the GPWS warning once we were airborne. I don't know why the GPWS warning activated after takeoff.


Synopsis: Air carrier flight crew reported experiencing a terrain warning on initial climb due to a malfunctioning GPWS.