Shortly after 3 a.m. on Tuesday, a B-2 Spirit, sometimes referred to as a “Stealth Bomber,” was forced to make an emergency landing in Colorado Springs after the failure of what seems to have been at least two integral flight systems. According to recordings of FAA traffic surrounding the incident, the advanced deep penetration bomber suffered a failure in one of its four engines as well as a failure in its communications system. The aircraft was unable to communicate with the tower at the airstrip it was forced to come down on and was instead guided down via handheld spotlights.

What caused these issues, however, remains unclear. The B-2 Spirit first took to the skies in the late 1980s but remains among the U.S. government’s most tightly held secrets, with nearly all flights of the aircraft traditionally conducted out of only four highly secure airstrips. For a B-2, which remains the most expensive aircraft (per unit) ever produced thanks to its advanced stealth capabilities, landing at a civilian airstrip where it would then remain on display for some time is more than unusual. Images of the incident soon made their way to the popular Facebook page Air Force amn/nco/snco.

The B-2 is equipped with four General Electric F118-GE-100 non-afterburning turbofan engines that are tucked deep within the aircraft’s wing shape. Burying the engines within the body of the B-2’s flying wing shape reduces the aircraft’s radar signature as compared to bombers with external engines on their wings, while also providing an opportunity to conceal some degree of thermal visibility. Cold air is actually funneled through the aircraft via an opening just below each engine’s main inlet. That cold air flows directly into the concealed exhaust ports at the rear of the aircraft, reducing the overall exhaust temperature and, as a result, the likelihood that anti-aircraft assets or intercept fighters could lock on to any sort of infrared signature.

However, flying wing designs — particularly one designed with concealed internal engines like the B-2 — are less stable after the loss of a single engine than traditional aircraft. While it’s not unheard of for conventional bombers to lose one or even two engines and still manage to return to their airstrips, the loss of one of four engines in a B-2 could make the aircraft unwieldy and extremely difficult to operate. If two engines were to fail on the same side of the aircraft, maintaining any kind of control could become too difficult to manage at all.

Unconfirmed reports from the area say that one of the pilots appeared to have been treated with oxygen by emergency crews upon the bomber’s landing. If true, such a treatment could potentially indicate that there were other system failures aboard the aircraft or even smoke inside the cockpit. However, because of the stressful and harrowing circumstances of averting disaster in a billion dollar state secret over a populated U.S. city, it’s possible the pilot was simply being treated as a precautionary measure.

Earlier this year, another of the U.S.’s long-serving bombers, a B-1B Lancer, also suffered an in-flight emergency that prompted the four-man crew to attempt to eject. When one of the ejection seats failed, however, the crew managed to land the aircraft safely, prompting an investigation into each of the series of failures that placed the training flight in such jeopardy. The U.S. military has been plagued by aircraft failures and mishaps in recent years, with most attributed to increased operational requirements and decreased maintenance tied to funding issues. Nonetheless, these types of incidents involving America’s bombers, particularly its stealth bombers, are extremely uncommon.

The B-2 is slated to be replaced by the forthcoming B-21 Raider starting in the mid-2020s, but these legacy bombers are expected to remain in service for decades to come as production of the B-21 continues.