Piper L 21B accident near Big Timber, Montana, July 8, 2017
On July 8, 2017 at about 4:45 pm local time, a 1954 Piper L 21B, registered N99031, was substantially damaged in an accident during maneuvering (low-alt flying) near Big Timber, Montana. It was a personal flight under general aviation rules (Part 91). 2 people were seriously injured. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
The pilot's decision to fly at low altitude in the vicinity of power lines, which resulted in a wire strike.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- July 8, 2017 · about 4:45 pm local time
- Place
- Big Timber, Montana · map
- Type
- Accident
- Injuries
- 2 people were seriously injured.
- Weather
- visual conditions (good weather)
- Aircraft
- Piper L 21B B, built 1954 · all L 21Bs on the register
- Registration
- N99031 · registry record · serial 54-2590
- Damage
- Substantial damage
- Flight
- Personal flight · general aviation rules (Part 91)
The NTSB's narrative final · quoted from the NTSB record
The private pilot reported that, after departure, he followed the course of a river at an altitude about level with the treetops. Although the pilot was aware of powerlines in the area, he did not see them before the airplane collided with the powerlines about 90 ft above ground level. The airplane subsequently nosed down and impacted the river, resulting in substantial damage. According to the manager of the airport from which the airplane departed, it is not uncommon for pilots to fly at low altitudes in that area to avoid turbulent air in the summertime. He further reported that the power lines were not equipped with aerial marker balls. Following the accident, the airport manager agreed to install signage in the fixed base operator at the airport to caution pilots of the presence of power lines in that area.
The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
What happened, in order
- Collision with terrain or object (not controlled flight into terrain) during maneuvering (low-alt flying)
- Loss of control in flight during maneuvering (low-alt flying) defining event
- Collision with terrain or object (not controlled flight into terrain) during uncontrolled descent
The NTSB's findings
- cause Personnel issues › Action/decision › Info processing/decision › Decision making/judgment › Pilot
- cause Personnel issues › Psychological › Attention/monitoring › Monitoring environment › Pilot
- cause Aircraft › Aircraft oper/perf/capability › Performance/control parameters › Altitude › Not attained/maintained
Pilot
- Certificate: private
- Ratings: single-engine land
- Flight time: 600 hours in all; 180 in this make and model; 2 in the last 90 days; 585 as pilot in command
- Medical certificate: Class 3 (with waivers/limitations)
- Seat: frt
- Injury: serious injuries
The aircraft
- Airframe total time: 2,557.2 hours
- Last inspection: annual inspection, June 28, 2017; 0 hours since
- Seats: 2
- Landing gear: fixed
- Engine: Lycoming O-320-A2B (piston); 102 hours total
The flight
- Departed from: 6S0 Big Timber MT at 4:30 pm
- Destination: 6S0 Big Timber MT
- Flight plan: none
Weather at the time
- Light: daylight
- Wind: from 220° at 11 knots
- Visibility: 10 statute miles
- Sky: clear
- Temperature: 77°F (25°C), dew point 52°F (11°C)
- Altimeter: 30.20 inHg
- Observation at 4:55 pm from 6S0, 4 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 1 | |||
| Passengers | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
5 documents, released by the NTSB on September 1, 2017. View them here, or download them; the NTSB redacts some personal details before release, and this site shows the NTSB's own titles rather than its file names.
| # | Document | What it is | |
|---|---|---|---|
| 1 | Pilot/operator Aircraft Accident Report, NTSB Form 6120.1 | PDF, 9 pages | View Download |
| 2 | Record of Conversations | PDF, 4 pages | View Download |
| 3 | Weather Report and Records | PDF, 2 pages | View Download |
| 4 | Sun Azimuth, U.s. Naval Observatory | PDF, 3 pages | View Download |
| 5 | Accident Photographs | PDF, 4 pages | View Download |
The same docket at the NTSB · documents without a copy here are fetched from the NTSB when you open them.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.